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IC ®®^^ 



Bureau of Mines Information Circular/1982 



) > 



Underground Coal Mine Power Systems 

Proceedings: Bureau of Mines Technology Transfer 
Seminar, Pittsburgh, Pa., September 16, 1982 



Compiled by Staff-Bureau of Mines 



UNITED STATES DEPARTMENT OF THE INTERIOR 



Information Circular 8893 



Underground Coal Mine Power Systems 

Proceedings: Bureau of Mines Technology Transfer 
Seminar, Pittsburgh, Pa., September 16, 1982 



Compiled by Staff-Bureau of Mines 




UNITED STATES DEPARTMENT OF THE INTERIOR 
James G. Watt, Secretary 

BUREAU OF MINES 
Robert C. Norton, Director 



^\\ 



A^ 






<^^13 




This publication has been cataloged as follows: 



Bureau of Mines T( 


chnology T 


ran- 


fer Seminars 


(1982 : 


Pittsburgh, Pa.) 










Underground coal mine power syste 


TIS. 






(Information circular 


; 8893) 








Includes bibliographi 


cal references. 








Supt. of Docs, no.: I 


28.27:889 3. 








1. Coalmines and min 
2. Electricity in mining 
Mines. II. Title. III. 
Bureau of Mines) ; 8893. 


ing — United States— Power Supply — Congresses. 
— (Congresses. 1. United States. Bureau of 
Series: Information circular (United States. 


TN295.U4 [TN343] 


622s [622' 


.48] 


82-600227 





f^ 



PREFACE 

This Information Circular sununarizes recent Bureau of Mines research results con- 
cerning improved mine electrical power systems for America's underground coal mines. 
The papers are only a sample of the Bureau's total effort to improve coal mine safe- 
ty, but they do delineate the major concerns of the mine electrical power programs. 
Some of the technology discussed has applications in other types of mining. 



The six technical presentations reproduced here were made either by Bureau 
^"Researchers or by personnel representing Bureau contractors at the Technology Trans- 
fer Seminar on Underground Coal Mine Power Systems given in September 1982 in Pitts- 
burgh, Pa. The content of the other presentation not included here can be found in 
the Bureau of Mines Handbook, "Application Notes — ^Mine Electric Power Systems." 
\ Those desiring more information on the Bureau's mine electrical power safety programs 
in general, or information on specific situations, should feel free to contact 
the Bureau of Mines Division of Health and Safety Technology, 2401 E Street, NW, 
Washington, D.C., 20241, or the appropriate author. 



^ Was] 



^^ 



^b^O 



CONTENTS 



iii 



Page 



Preface 1 

Abs tract 1 

Introduction 2 

Design Practices To Minimize the Probability of Shock During Control Box 

Maintenance, by Thomas Novak and George J. Conroy 4 

A Design Guide for Explosion-Proof Electrical Enclosures, by P. A. Cox and 

Lawrence W. Scott 11 

High-Voltage, Explosion-Proof Load Centers, by George Conroy, Randy Berry, 

and Robert Gillenwater 29 

Demonstration of the Discriminating Circuit Breaker (DISCB), by 

Michael R. Yenchek 55 

Intermittent Duty Rating of Trailing Cables, by George J. Conroy and 

Herman W. Hill 74 

Semiconducting Rubber as a Low-Voltage Shield for Personnel Protection, by 

J. N. Tomlinson and L. A. Morley 82 



UNDERGROUND COAL MINE POWER SYSTEMS 

Proceedings: Bureau of Mines Technology Transfer Seminar, 
Pittsburgh, Pa., September 16, 1982 

Compiled by Staff-Bureau of Mines 



ABSTRACT 

This Bureau of Mines publication presents an overview of mine electri- 
cal power systems research currently being conducted by the Bureau. The 
papers, given at a Technology Transfer Seminar, emphasize the increas- 
ing importance of research related to the safety considerations of 
underground coal mine electrical systems. Selected topics are included 
that summarize results of research in the areas of electrical shock 
prevention, explosion-proof enclosures and load centers, discriminating 
circuit breakers, and trailing cables. Other topics discussed at the 
seminar are published as separate sections of the Bureau of Mines Hand- 
book "Application Notes — Mine Electrical Power Systems." 



INTRODUCTION 



Mine power systems (MPS) research 
encompasses the following areas: per- 
missible equipment, power distribution, 
dc power systems, cable usage problems, 
and intrinsic safety. In general terms, 
MPS research attempts to identify and 
attack electrical problems which pose a 
significant hazard to the safety of the 
individual miner. This is accomplished 
by one of several methods. 

In studying shock prevention, for exam- 
ple, the problem must be correctly iden- 
tified. To this end. Mine Safety and 
Health Administration (MSHA) accident 
statistics were studied to build a 
history of electrical accidents both 
fatal and nonfatal; then innovative but 
practical solutions were proposed and 
evaluated. Some of these solutions are 
contained in the first paper. Shock pre- 
vention research will continue to receive 
a high priority in future MPS research. 

A recent MSHA publication^ attributes 
60 pet of the mine fires investigated 
through 1972 and 21 percent of all meth- 
ane ignitions to electrical ignition 
sources and inadequate design of electri- 
cal equipment. A Bureau study2 of elec- 
trical violations cited by MSHA indicated 
that the second most common electrical 
violation was failure to comply with reg- 
ulations for permissible electrical face 
equipment maintenance. In fact, the 
improper use of permissible equipment 
accounted for 17 pet of the total elec- 
trical violations studied. 

The study of permissible equipment 
and enclosures is an area requiring 
investigation into materials, methods 
of construction, and testing criteria. 
At present, the explosion-proof enclo- 
sures used in underground mines are 
constructed to rigid design requirements 
that contribute to the difficulties in 

""msHA IR-1018, "Electrical Hazards in 
Underground Bituminous Coal Mines," 1975, 
5 pp. 

^BuMines IC 8725, "Mine Inspection Rec- 
ords Study," 1976, 11 pp. 



maintaining the enclosures in a permissi- 
ble condition. If designers were per- 
mitted more freedom, it would be possible 
to construct enclosures that are easier 
to maintain in a permissible condition, 
and to eliminate aluminum alloys if they 
are deemed a safety hazard. These new 
enclosures would be submitted to perform- 
ance tests to insure that they offer the 
same degree of safety as the enclosures 
presently being constructed to the design 
requirements. It is the objective of 
this research area to determine the 
safety factors in the present design 
requirements, and to develop and demon- 
strate the feasibility of performance 
tests. The second paper deals with the 
results to date of this research area. 
Another paper deals with the need for 
ever-increasing power requirements of 
larger and larger face equipment. The 
paper on explosion-proof load centers 
presents tentative specifications and a 
request for interested persons to 
comment . 

At least 80 of the 127 coal mine fires 
involving the trolley system, which were 
investigated by Federal personnel from 
1952 to 1977, could have been prevented 
if circuit protection systems capable of 
responding to low-current arcing faults 
had been available and universally in- 
stalled. If one considers only non- 
reportable fault conditions on haulage- 
ways, the annual worth of a low-level 
fault protection scheme is estimated at 
$21,000 to $52,000 per mine. Thus, even 
on a cost-saving basis, without mention- 
ing saved lives, sensitive trolley wire 
protection can be shown to be of value to 
mine operators. This does, however, 
require physical demonstration that the 
method performs as required. To meet 
this objective, the Bureau sponsored 
research that led to the development of 
the discriminating circuit breaker 
(DISCB) described in the fourth paper. 
This device may find future applications 
in electrified mass transit systems. The 
DISCB can distinguish between low level 
ground fault currents and legitimate 
high-current loads on a mine haulageway, 



responding (instantaneously) only to the 
former. It can be employed in any exist- 
ing haulageway with a minimum amount of 
modification of the haulage equipment, A 
scaled demonstration is described in the 
paper. 

Cables contribute more to downtime 
and to personnel injury than any other 
electrical component used in mines. 
Accident classification by injured 
activity from 1975 to 1979 has shown 
that in 19 pet of all accidents contact 
with energized cables occurs while 
splicing a supposedly dead line. Proper 



ratings, construction, and application of 
cables is very important. The last two 
papers discuss these subjects in more 
detail. 

The increased use of electrical power 
in the mining environment dictates that 
research into specific mine electrical 
hazards be focused on those areas which 
will most significantly reduce fatal and 
nonfatal accidents among miners. MPS 
research will continue to identify elec- 
trical hazards and propose innovative, 
yet practical, solutions to alleviate 
those hazards. 



DESIGN PRACTICES TO MINIMIZE THE PROBABILITY OF SHOCK 
DURING CONTROL BOX MAINTENANCE 

By Thomas Novak! and George J. Conroy2 



ABSTRACT 



The use of dead-front construction and 
other means of segregating higher-voltage 
portions of control box circuitry with 
carefully planned use of test points, 
an in-place schematic diagram, and indi- 
cator lights, are explained in this 
paper. Sensitive circuitry for personnel 



protection is discussed. The reception 
by manufacturers and mine personnel of 
the ideas as embodied in a demonstrator 
unit is summarized. Recommendations to 
improve maintenance safety are listed and 
discussed. 



INTRODUCTION 



Mine Safety and Health Administration 
(MSHA) accident statistics for both coal 
and metal-nonmetal mines show that main- 
tenance personnel have the highest fre- 
quency of electrical accidents. These 
statistics might be expected since re- 
pairworkers have the greatest exposure to 
electrical hazards. Yet electrical- 
safety features of equipment have been 
directed primarily toward protecting the 
electrical wiring, the machine, and the 
mine from heat and fire damage. Consid- 
ering the evidence, present emphasis 
should be placed on protection of mainte- 
nance personnel. 

The harsh operating conditions common 
to most mining processes contribute 
greatly to the electric shock hazard. As 
mineral or rock is extracted, the elec- 
trical machines must advance, followed by 
their sources of power. During these 
moves, both equipment and cables are fre- 
quently stressed by pulling over rough 
surfaces and being twisted or impacted. 
As all those who work in mines can 
attest, gentle handling of equipment is 
not the rule but the exception. The 
extreme abuse increases the amount of 

"• Professor of electrical engineering, 
Pennsylvania State University, Fayette 
Campus, Uniontown, Pa. 

^Supervisory electrical engineer (re- 
tired). Bureau of Mines, Pittsburgh Re- 
search Center, Pittsburgh, Pa. 



maintenance while at the same time it 
renders insulating qualities, etc. , ques- 
tionable, all of which increases the 
repairworker's exposure to electric 
shock. 

The environmental conditions of the 
mine also enhance the probability of 
electrical accidents. Wet conditions are 
often encountered which decrease the con- 
tact resistance between a person and 
earth. Thus, a person becomes more sus- 
ceptible to electrocution from contacting 
an energized conductor. 

Space on mining machines is extremely 
limited, and electric control boxes and 
panels are crowded with parts. Trouble- 
shooting with the circuit energized is 
permitted under present regulations, 
leading to a distinct probability that, 
either through ignorance, inattention, or 
an Inadvertent slip, an elbow or hand 
will contact an energized member. 

If normal production is interfered with 
by a breakdown, which is frequently the 
case, the repairworker will probably be 
rushed. Especially in mines where roof 
and rib might be unstable because the 
machine cannot do its roof-bolting job, 
the cramped working conditions and the 
foreman's frequent encouragement to com- 
plete the repair, can create sufficient 
confusion to add to existing potential 
hazards. 



There is not much hope that the working 
conditions will change; nevertheless, the 
accident statistics can be improved. 
Electric shock prevention can be afforded 
to the maintenance worker as follows: 

1. Minimizing exposure to energized 
members. 

2. Providing sensitive, rapid-response 
electrical protective devices. 

Both of these approaches will be 
discussed. 



The first approach can be implemented 
by having the machine manufacturer or, in 
some cases, the mine maintenance group 
itself, incorporate certain physical- 
mechanical design modifications that are 
simple in concept but so beneficial in 
their consequences that their added first 
cost can easily be shown to be balanced 
by reduced maintenance costs and improved 
safety. The second approach is, in some 
cases, only now becoming fully realiz- 
able, with the entry into the mining 
field of rugged, reliable solid-state 
devices. 



PHYSICAL DESIGN MODIFICATIONS 



The design concepts to be considered in 
this paper are dead-front construction, 
interlocks, segregation of high-voltage 
circuits, and lockouts. They may be 
employed singly or in combination, with 
the objective of avoiding inadvertent 
contact with energized members by intro- 
ducing elements which, while restraining 
from contact, facilitate or at least do 
not significantly hinder rapid trouble- 
shooting. The designer should also give 
consideration to secondary aspects of 
protection, as explained when discussing 
interlocks which remain, even when the 
primary protective elements are bypassed. 

Dead-Front Construction 

It is possible to perform a large num- 
ber of troubleshooting operations with- 
out making a manual approach of any kind 
to the vicinity of the high voltage 
(600 V, 480 V, etc.) portions of a con- 
trol circuit, if test points are supplied 
on a panel that is interposed as a phys- 
ical barrier between the circuit and 
the external world. For application to 
explosion-proof enclosures, a hinged pan- 
el can be installed in a position slight- 
ly recessed behind the outer cover of the 
enclosure. The test points are installed 
to penetrate the surface of the hinged 
panel to allow important voltage measure- 
ments. The concept requires little or no 
increase in overall size of the control 
case. 



The dead-front, troubleshooting concept 
can be applied to almost any type of 
electrical-control case. A simplified 
unit was constructed to demonstrate the 
concept and is shown in figure 1. The 
internal components comprise a simple, 
two-motor starting circuit. Since this 
prototype was built for use as a demon- 
stration unit, the electrical con^jonents 
are housed in a portable, lightweight, 
aluminum enclosure rather than a heavy, 
explosion-proof box; however, the ulti- 
mate usage was kept in mind at all times. 

The unit is designed to simplify 
troubleshooting procedures, as well as to 
reduce the hazards of electrical shock. 
Figure 1 shows the dead-front panel with 
the outer lid of the enclosure removed. 
The incorporation of test points into an 
actual diagram of the circuit schematic 
is an important aspect. This feature 
could greatly simplify troubleshooting 
techniques and possibly reduce machine 
downtime. In addition, the schematic 
provided on the panel is more permanent 
than paper schematics, which deteriorate 
quickly in the moist mine atmosphere and 
are susceptible to being mislaid and 
lost. The hinged panel provides a com- 
plete barrier between the repairworker 
and the electrical components. The test 
points are pin jacks and therefore re- 
strict access to dangerous voltages re- 
quiring the use of test probes. A key is 
required to gain access to the internal 




FIGURE 1. - Dead-front panel with outer lid of 
enclosure removed. 



FIGURE 2. - Panel as removed after internal 
electrical controls are unlocked. 



electrical controls. After being un- 
locked, the panel can be opened, as shown 
in figure 2; however, opening the panel 
results in the operation of two interlock 
switches which are in series with the 
undervoltage release of the circuit 
breaker. The opened interlock switches 
interrupt power to the undervoltage re- 
lease, tripping the circuit breakers. 
Thus, all internal components, with the 
exception of the incoming leads of the 
circuit breaker, are deenergized. A 
guard with a warning light is placed 
around the incoming connectors of the 
breaker to prevent accidental contact 
should the repairworker fail to deener- 
gize the incoming line. On an actual ac 
machine used in coal mining, the inter- 
lock switches would usually be connected 
in series with the pilot circuit of the 
ground-check system rather than to an 
internal circuit breaker. Opening the 
panel would trip the upstream circuit 
breaker, located in the section power 
center; thus all power to the control 
case would be shut off. In that appli- 
cation, the warning light would be re- 
tained, whether or not the internal cir- 
cuit breaker was included. 

Many variations of the basic dead- 
front concept can be applied to control 
centers. One possibility is to use a 
dead-front panel made of transparent 
plastic, such as polycarbonate. Since 
the electrical controls are located 



directly behind the panel, the physical 
movement of line starters, relays, and 
contactors can be observed while voltage 
measurements are made at the isolated 
test points. The transparent panel can 
be so constructed that permissibility is 
not compromised, with only intrinsically 
safe test points used if any are needed. 
With this design, a relatively thin, 
steel-hinged outer cover protects the 
plastic panel from most sources of 
scratching or other damage. Whether or 
not the transparency of an unprotected 
panel can be maintained under actual 
operating conditions depends upon the in- 
dividual application. 

A further refinement of the dead-front 
concept is to replace or supplement the 
test points with visual indicators, such 
as LED's or lamps. In effect, the built- 
in indicators would serve in place of a 
voltage tester for many routine trouble- 
shooting observations, and a quick 
assessment of the condition of the con- 
trol circuit could be accomplished simply 
by visual inspection of the panel. 

Testing and changing control-circuit 
fuses are among the most frequent 
electrical-maintenance procedures. For a 
dead-front approach, the fuses can be 
mounted behind a separately interlocked 
explosion-proof cover. The interlock 
would operate an internal contactor 
which, because of its very low duty cycle 



with regard to circuit interruption, 
could be relatively small. The fuses 
could thus be removed and replaced with 
no risk of having personnel contact ener- 
gized metal clips. A refinement of this 
idea is to include test points under the 
interlocked cover connected to each end 
of each fuse. With the circuit opened by 
the interlock, fuse continuity can be 
checked with an ohmmeter without removing 
the fuse. 

Interlock Disable Precautions 

Interlock design should always be ori- 
ented toward prevention of accidental 
reclosure of the interlock. Where the 
interlock is a simple jump-link arrange- 
ment, the energized contacts should be 
female, recessed in insulating plastic. 
If a sensitive switch is used, the pres- 
sure to close the switch should be ap- 
plied by a protrusion operating through a 
hole in the panel behind which the switch 
is mounted. Any safety feature can be 
circumvented by a determined maintenance 
worker if it interferes with his or her 
troubleshooting function. Since some 
interference by an interlock is unavoida- 
ble, the means to circumvent the device 
should be thought out beforehand by the 
designer and not left to be jury-rigged 
in a manner that permanently damages or 
disables the interlock. In its most 
desirable form, the disabling means will 
automatically clear once the necessity 
for its use is over. An example would be 
a latched interlock switch arranged so 
that cover closure results in mechan- 
ically resetting of the latch. Also, a 
highly visible warning should be included 
telling when an interlock has been dis- 
abled, to remind the repairworker that 
circuits remain energized. 



point on a terminal strip must be num- 
bered to correspond with the circuit's 
schematic, and all con^jonents within the 
case should be labeled for easy identifi- 
cation. The test points should also be 
segregated according to their voltages. 
In other words, control-circuit test 
points of 120 V should be located in an 
area away from the power-circuit test 
points of 440, 550, or 950 V. The re- 
pairworker would then know the approxi- 
mate level of the voltage with which he 
or she is dealing. The color coding of 
conductors might also be helpful in dis- 
tinguishing different voltage levels. 

The main disadvantage of using isolated 
test points or voltage indicators for 
troubleshooting is that the amount of 
wiring is increased. The additional wir- 
ing increases as the size of the circuit 
increases. With very large circuits it 
may not be feasible to isolate all possi- 
ble test points. A circuit analysis 
would have to be performed to determine 
which test points are the most critical. 
The critical test points would then be 
brought out to the dead-front panel or 
terminal strips. 

The preceding discussions primarily 
deal with machine circuitry. However, 
the concepts are not limited to this 
application. Dead-front, troubleshoot- 
ing panels can also be applied to 
the low- and medium-voltage circuits of 
power-distribution equipment in a similar 
fashion. 

Lockout Features 



The following example illustrates a 
maintenance-related electrical accident 
that has occurred numerous times: 



Segregation of Different Voltages 

For some controls, isolation of the en- 
tire circuit with a physical barrier may 
not be possible or desirable. One means 
of improving electrical safety in these 
situations is to include one or more ter- 
minal strips for use as test points. The 
terminal strips should be located such 
that accidental contact with energized 
conductors would be minimized. Each test 



A repairworker deenergizes the faulty 
circuit at the power center prior to per- 
forming repair work. While he or she 
works on the faulty circuit, another 
worker, who is unaware of the situation 
or mistakes the cable of the faulty cir- 
cuit for another cable, energizes the 
faulty circuit and thus subjects the 
repairworker to electrical shock or 
electrocution. 



Mine power center manufacturers have 
made available lockout features on cable 
couplers to prevent this situation. 
Locking-type dust covers on equipment- 
mounted receptacles along with keyed 
couplers are supplied as a means of re- 
ducing the possibility of this hazard. 
With keyed couplers, the plug of each 
outgoing circuit is matched to fit only 
one receptacle. The dust covers are con- 
nected to the coupler or the equipment by 
a chain in order to prevent their loss 
while not in use. A loosely hinged cover 
would serve the same purpose with in- 
creased operational facility and greater 
assurance against damage. 

Locking-type dust covers for the cable- 
mounted plug can afford even more protec- 
tion for maintenance personnel. Once the 
plug is locked, connection to any recep- 
tacle is impossible. Chain-connected, 
locking-type dust covers are presently 
available for high voltage plugs. How- 
ever, consideration should be given to 
the development of hinged dust covers for 
all plugs energized at higher than 30 or 
40 V. 

Electrical lockout features utilizing 
the ground monitor circuit can also be 
incorporated to give the mechanic working 
at the machine the ability to prevent 
reestablishment of power from the load 
center, until he or she is fully ready to 
allow it. A typical circuit would employ 
an interval timer, a small relay, and a 
reset switch. 

When lockout features — and strict 
procedures for their employment — are 
combined with other cable and machine- 
related safety provisions, a very signif- 
icant improvement in the count of fatali- 
ties and injuries may be observed. 

Sensitive Electrical Protection 

This subject will be summarized here 
rather than explored in detail, as a very 
thorough treatment will become available 
in the final report of a Bureau contract 
(J0113009) during November 1982. Many 
aspects of the foregoing discussion are 
also included in the report. The present 



discussion will center on the physiologi- 
cal basis for relying on sensitive elec- 
trical protection and will simply mention 
a few of the findings of that study and 
other research projects. 

Electric Shock Threshold 

Ventricular fibrillation is usually the 
most dangerous shock hazard, as it occurs 
at relatively low values of current. 
Disabling or fatal burns require larger 
currents, and cardiac arrest due to high 
current flow can, in some cases, be coun- 
tered by appropriate physical action by 
another person after the current has been 
turned off. When fibrillation occurs the 
pumping motions of the heart become dis- 
organized and, finally, the pulse ceases. 
Death occurs within minutes. Dalziel 
(_1_)3 presented an equation extrapolating 
statistical data obtained from experi- 
ments on animals, which predicts the min- 
imum threshold for fibrillation for a 
body weight of 110 lb (50 kg) at a power- 
line frequency of 60 Hz 

I = 8.3 msec < t < 5 sec, 

ft - - 

where I = minimum current in milli- 
amperes through major 
extremities. 



and 



t = duration of the shock, in 
seconds. 



The equation is represented graphically 
by the solid curve of figure 3. Accord- 
ing to Dalziel (J_) , the area underneath 
and to the left of the solid curve is 
considered the "safe area" with respect 
to fibrillation. 

Although it is impossible to eliminate 
fatal reactions to electrical shock for 
all cases, a reasonable degree of safety 
can be achieved by limiting the maximum 
ground-fault current to a low value (say 
500 mA) and by matching the characteris- 
tics of a ground-fault relay to the 

■^Underlined numbers in parentheses 
refer to items in the list of references 
at the end of this paper. 





1 


r- 1 1 


- 




Fibrillation curve 


- 


\ 


Operating characteristic for 
a definite-time relay 




-\ 


Fibrillation 


Pick- 
up 


Safey 
\area 


"'x,^^ Operating time 


V 


^^ 


^ ^ 



100 200 300 400 500 
CURRENT, mA 

FIGURE 3, - Fibrillation curve. 

fibrillation curve. If the Dalziel equa- 
tion is accepted, a definite-time relay 
with an operating characteristic similar 
to the dashed line of figure 3 would be 
required to meet the necessary sensitiv- 
ity. The pickup current of 50 mA was 
selected since this value is generally 
considered the minimum threshold of fib- 
rillation. The operating time of 0.1 sec 
is based on worst-case conditions and 
takes into account system capacitance and 
the reaction time of molded-case circuit 
breakers. 

Ventricular fibrillation can also be 
caused by direct current, but related re- 
search has not been nearly as con5)rehen- 
sive when compared with alternating cur- 
rents. However, Daziel (O indicates 
that the fibrillation level for dc cur- 
rents is approximately five times the 
threshold of ac currents. Therefore, the 
pickup current of 250 mA at an operating 
time of 0.1 sec is suggested for dc 
applications. 



Ground Fault Current Interrupters (GFCI) 

The GFCI concept was originally devel- 
oped by Fuchs-Westinghouse, Ltd. 4 in 
South Africa, for use in residential wir- 
ing, and is presently commercially avail- 
able from a number of sources in the 
United States. It is being increasingly 
incorporated into single-phase systems in 
both residences and industrial buildings. 
The system depends on having a toroid 
with a "square" hysteresis loop, through 
which all power conductors pass, so that 
the flux created by the current out the 
one conductor can be balanced by the flux 
generated by return current in the other 
conductor. Any unbalance indicates the 
presence of a leakage current through the 
earth or external grounding circuits and 
if the output of a secondary winding on 
the toroid is utilized to trip a sensi- 
tive relay circuit, leakage currents as 
small as 5 mA will result in circuit 
interruption. 

In the investigation conducted under 
Bureau of Mines contract JOl 13009, no 
commercially manufactured GFCI was found 
for the high-current applications pecu- 
liar to mining. Generally, the highly 
sensitive response to fault currents was 
accompanied by a similarly high sensitiv- 
ity to electrical "noise" pulses con- 
ducted through the mine wiring or trans- 
mitted through space. Frequent nuisance 
tripping resulted, which would make the 
devices intolerable to operating person- 
nel. It is possible, however, to include 
devices of this type in control boxes, to 
be enabled when the outer cover is opened 
but bypassed in normal operation. In 
this way, the maintenance worker would 
have highly sensitive protection at the 
expense of susceptibility to nuisance 

'^Reference to specific manufacturers 
does not imply endorsement by the Bureau 
of Mines. 



10 



tripping, whereas the noise immunity of 
the machine would not be conqjromised dur- 
ing routine operation. 

Sensitive Earth Leakage (SEL) System 

The SEL system has been used for over 
10 years in the United Kingdom, and has 
been recently introduced into the U.S. 
mining operations by the Consolidation 
Coal Co., a subsidiary of DuPont Indus- 
tries {2). Full details of the funda- 
mental circuit have not been revealed. 
For U.S. mines, the British circuit 
was modified by adding an input filter, 
shielding the current transformer, and 



providing a short time delay, all to min- 
imize nuisance tripping. The modified 
system limits the maximum ground-fault 
current to 500 mA, while the relay picks 
up at 140 mA., operating about 0.17 sec 
(10 cycles of 60-Hz current) after initi- 
ation. While these values of fault cur- 
rent and time are not yet comparable to 
the safe levels indicated in figure 3, 
there is a strong promise that future 
improvements will result in a relaying 
system that matches the desired charac- 
teristic. Meanwhile, the use of the sys- 
tem improves the probability that perma- 
nent damage will not result from contact 
with energized members. 



CONCLUSION 



MSHA statistics show that maintenance 
personnel are the primary victims of 
electrical accidents in the mining in- 
dustry. This high incidence can be 
attributed to the worker's frequent ex- 
posure to electrical hazards. Reduced 
electrical exposure, through the use of 
dead-front construction and other physi- 
cal barriers between the control circuit 
and the troubleshooter, in combination 
with test points, interlocks, and circuit 
lockout features, can play a significant 
role in decreasing electrical accidents. 
Sensitive ground-fault relaying, which is 



capable of detecting and isolating a 
ground-fault prior to electrocution, 
should be developed to provide protection 
if preventive measures fail. Dalziel's 
(1) ventricular fibrillation curve could 
define the ultimate time-current char- 
acteristic of such a relaying system. It 
is hoped that the illustrated examples 
provided in this paper serve as a start- 
ing point in a campaign to enhance elec- 
trical safety and that new approaches 
suggest themselves as these few sugges- 
tions are tried. 



REFERENCES 



1. Dalziel, D. F. Electric Shock 2. Dolinar, K. D. Improved Ground 
Hazard. IEEE Spectrum, February 1973, Fault Protection System for Low and 
pp. 44-50. Medium Voltage Trailing Cables. IAS of 

lEEEProc. , 1982, pp. 122-127. 



11 



A DESIGN GUIDE FOR EXPLOSION-PROOF ELECTRICAL ENCLOSURES 
By P. A. Cox"! and Lawrence W. Scott2 



ABSTRACT 



A guide Is being prepared for the 
design of explosion-proof electrical 
enclosures. It will address those as- 
pects of design which affect enclosure 
strength and ruggedness. Material selec- 
tion will be covered only for materials 
such as polycarbonates, adhesives, and 
sealants which can be seriously degraded 
by the mine environment. The guide 



will not address electrical function, nor 
will it duplicate or supplant the re- 
quirements set forth in the Code of Fed- 
eral Regulations (CFR), Title 30, Part 18 
(Schedule 2G). If followed, the design 
guide should assure that the strength and 
ruggedness requirements of Schedule 2G 
are met. 



INTRODUCTION 



This paper represents a status report 
on the preparation of a design guide for 
explosion-proof electrical enclosures. 
It is proposed that the guide be pub- 
lished under the auspices of the Federal 
Bureau of Mines and that it be made 
available to both the Mine Safety and 
Health Administration (MSHA) and enclo- 
sure manufacturers. Use of the design 
guide will be optional. Its purpose is 
to provide guidelines for the design of 
explosion-proof enclosures which, if fol- 
lowed, will assure that the enclosure 
will both pass MSHA certification testing 
and fulfill its intended service life 
underground. The guide will cover only 
those aspects of design that affect en- 
closure strength, ruggedness, and service 
life. It will not address electrical 
function such as connector design, light- 
ing, electrical leads, or electrical pen- 
etrations. An attempt has been made to 
keep the guide general, rather than spe- 
cific, so that it will continue to be 
useful as MSHA requirements evolve. For 
example, ruggedness is not now a specific 
requirement in Schedule 2G. Ruggedness 

^Senior research engineer, Southwest 
Research Institute, San Antonio, Tex. 

^Technical project officer, Pitts- 
burgh Research Center, Bureau of Mines, 
Pittsburgh, Pa. 



has been achieved in the past by minimum 
thickness requirements and by design to 
withstand internal explosions of methane 
and air; however, future regulations may 
eliminate minimum thickness requirements 
and also permit the venting of enclosures 
to reduce internal overpressures. If 
this occurs, a ruggedness requirement may 
be imposed to insure that the enclosure 
will withstand the mine environment. 
Thus, a section on ruggedness will be 
included in the design guide. 

The concept for the design guide and 
much of the material in it originated 
under Bureau of Mines contract H0377052. 
This contract included a survey of design 
and fabrication practice in the manufac- 
ture of explosion-proof enclosures. The 
survey showed that a design guide could 
be useful to the mining industry to unify 
and formalize procedures already being 
followed and to introduce to the industry 
new data and procedures generated in this 
contract. Data are also being taken 
from other related research, such as 
Bureau contract H0387009 (4_)3 and from 
numerous other sources as cited in the 
bibliography of this paper. 

■^Underlined numbers in parentheses 
refer to items in the bibliography at the 
end of this paper. 



12 



ORGANIZATION AND CONTENTS OF THE GUIDE 



The guide is organized to address the 
various steps in the design process in 
the approximate order in which they usu- 
ally occur. It is divided into many 
chapters, each of which covers a separate 
topic. It is hoped that through this 
organization of material the designers 
can easily locate and use the parts that 
they need or follow step by step through 
the entire design process. To facilitate 
the use of the guide by designers, as 
well as by analysts, many of the design 
process steps will be given in graphical 
format. 

When the guide is first used, it is 
recommended that the designer review each 
chapter for content and then read care- 
fully or work through examples (to be 
presented in the guide). The examples 
will cover the procedures described in 
the guide and will acquaint the designer 
with proper use of the equations, graphs, 
and data. 

Because the guide will cover a number 
of different topics, some extensively, 
not all of the material can be presented 
in this paper. Instead, a brief review 
of the contents of the guide by topic is 
given, and following sections give de- 
tails on three of the topics: "Design 
Pressures," "Design for Static Pressure," 
and "Design for Dynamic Pressure." 
Graphical solutions, which will be part 
of the completed design guide, have not 
yet been developed. 

Design Requirements . - This chapter 
will cite MSHA requirements that affect 
the strength of explosion-proof enclo- 
sures and discuss new requirements that 
are being considered for adoption by MSHA 
at the time the guide is published. MSHA 
requirements are subject to change, and 
current regulations must always be 
checked. 

Design Pressures . - This chapter will 
give procedures for choosing static and 
dynamic design pressures. Usually the 
static pressure will control the design. 



but dynamic effects must be checked also. 
Methods will be given for estimating the 
maximum value, rise time, and decay time 
of the dynamic pressure. The design pro- 
cedure assumes that pressure piling does 
not occur, and guidelines will be given 
that will help the designer avoid pres- 
sure piling in his or her enclosure. The 
"Design Pressures" section of this paper 
gives the major details of this chapter 
except for those which pertain to pres- 
sure piling. 

Design for Static Pressure . - Proce- 
dures will be given for sizing rectan- 
gular plates, circular plates, and cyl- 
inders to withstand the design static 
pressures without excessive distortion. 
Much of this chapter is included in this 
paper. 

Design for Dynamic Pressure . - Proce- 
dures will be given for checking dynamic 
effects of the loading for both elastic 
and elastic-plastic material behavior. 
For small deformations, such as now 
permitted by MSHA, the elastic proce- 
dure can be used with good accuracy even 
though plastic behavior is ignored. 
For larger permanent deformations, the 
elastic-plastic procedure is recommended. 
This material is covered in the related 
section of this paper. 

Design for Ruggedness . - MSHA does not 
now have an explicitly stated ruggedness 
requirement; however, a ruggedness test 
is being considered for adoption. This 
chapter will contain procedures for de- 
signing the enclosure to withstand exter- 
nal impact loads that can occur in the 
mine and which will most likely be in- 
cluded in a ruggedness requirement if one 
is adopted. 

Guidelines for Windows and Lenses . - 
This will be an extensive chapter 
that covers the design of windows and 
their mounting arrangements for use in 
explosion-proof enclosures (9^). Design 
procedures will be given for both glass 
and polycarbonate windows and lenses. 



Thermal Stresses in Windows. 



This 



brief chapter will give procedures for 
evaluating thermal stresses that are pro- 
duced in glass and polycarbonate windows 
and lenses by the MSHA thermal shock test 
with one-side quenching. Window mounting 
arrangements for minimizing thermally 
induced stresses will also be given. 

Welded Connections . - Weld joint design 
and joint efficiency will be covered in 
this chapter. Material is taken primar- 
ily from American Welding Society (AWS) 
Welding Code DW14.4, and this code is 
referenced for welding procedures, welder 
qualification, and weld joint inspection. 

Bolted Connections . - This chapter will 
contain design procedures for bolted con- 
nections, primarily for enclosure covers. 
Cover restraint associated with external 
loads and prying action produced by in- 
ternal loads will be treated. Connec- 
tions will be designed to avoid permanent 
deformation in the bolts which could in- 
crease the flange gap. 



13 



Reinforcement of Openings . - Simple 
formulas for area replacement and stiff- 
ness continuity will be given for use in 
designing reinforcement around openings 
in the enclosure. 

Materials of Construction . - This chap- 
ter will list materials that are suit- 
able for explosion-proof enclosures. It 
will include metals, polycarbonates, 
glasses, and adhesives. Service life re- 
strictions will be noted where applica- 
ble, and procedures will be given for 
qualifying new polycarbonates, adhesives, 
and sealants. 

Examples . - Examples that demonstrate 
the use of the material in the guide will 
be included in this chapter. These 
examples will not cover the steps a 
designer must follow which lead up to the 
selection of an enclosure geometry, but 
they will address the design details 
that assure proper enclosure strength. 



DESIGN PRESSURES 



Static Pressures 

Unless higher than expected pressures 
occur during the explosion test, the 
structural performance test will subject 
the enclosure to 150 psig. Because the 
magnitudes of pressures that can result 
from pressure piling cannot be predicted, 
it is proposed that the enclosure be 
designed under the assumption that pres- 
sure piling will not occur and that the 
guidelines for avoiding pressure piling 
(to be given in the design guide) will be 
followed. Using this approach, the de- 
sign static pressure will be 150 psig. 



from the work of Perlee (7^). To analyze 
the response of an enclosure for the ex- 
plosion pressure, the pressure's mag- 
nitude, rise time, and duration must be 
known. Methods for estimating the mag- 
nitude and rise time of the pressure are 
contained in following sections. Because 
the enclosure is usually tightly closed 
during the explosion test and in service, 
the pressure does not decay rapidly; 
therefore, its duration can be treated 
as long relative to the response time of 
the enclosure. 

Peak Pressure 



Dynamic Pressures 

Dynamic pressures are produced in an 
enclosure during the explosion test. 
These pressures are routinely measured by 
MSHA during testing and have also been 
measured by other investigators. For a 
spherical enclosure, a typical pressure- 
time history is given by Zabetakis ( 12 ) 
and shown in figure 1. Conqjarison be- 
tween experiment and theory is also given 



The peak pressure that theoretically 
can occur in a closed volume from the 
ignition of a mixture of methane and air 
at the stoichiometric4 ratio is 117 psig. 



'^Ratio at which there is, theoreti- 
cally, just sufficient oxygen in the air 
for complete combustion of all of the 
methane. This ratio is 9.5 pet for meth- 
ane and air. 



14 



120 



100- 



o) 80 — 



60- 



40 - 



20- 



I \ r 

Bulletin 627 



KEY 

Calculated 
Experimental 



10 20 30 40 50 60 70 
TIME, msec 





uu 
80 




60 


C3) 
CO 


40 


Q. 




n" 




Q- 


20 


o^ 




UJ 


10 




8 


CO 


6 


CO 




LU 
CC 


4 


a. 





1 1 




1 1 \j^_ 


- Rl 7839 




(J ~ 


" 


n 


= 4.3 A^ 






// 


— 


/ 


/ — 


- 


^ 


KEY 


y 


/ 


• Calculated 
o Experimental 


1 // 1 




1 , ,J 1 — L_ 



8 10 



20 30 40 50 70 

TIME, msec 



FIGURE 1. - Pressure in a 9-liter spherical chamber produced by ignition of a 9.6 voi-pct CH^-ai 
mixture. 



This pressure is produced by complete 
combustion and assumes that no heat loss 
to the walls of the enclosure and, of 
course, no pressure piling occur. A more 
realistic upper limit for the pressure is 
the measured value shown in figure 1. 
This pressure is for a 9.6 vol-pct 
methane-air mixture, which is slightly 
rich and in practice gives the highest 
measured value of the explosion pressure. 
Also, this pressure was measured in a 
spherical chamber with central ignition, 
which is an idealized condition relative 
to most enclosure tests. Typical values 
of pressure measured by MSHA in the 
explosion test are 60 to 80 psig. Thus a 
peak explosion pressure of 100 psig is 
suggested as a reasonable design value 
unless the designer has valid reasons for 
increasing its magnitude. For example, 
the dynamic design value might be in- 
creased if the particular geometry of the 
enclosure is expected to produce pressure 
piling. 



Rise Time 

The rise time of the explosion pres- 
sure, as well as its magnitude, is needed 
to calculate the response of the enclo- 
sure to these dynamic loads. Further, a 
short rise time generally produces a 
greater response in the enclosure than a 
long one. Therefore, an estimate of the 
minimum rise time is made for design 
purposes. 

To determine the minimum rise time, it 
is convenient to use the maximum rise 
rate of the pressure. Again, the data in 
figure 1 for spherical enclosures will be 
used. Using the minimum rise rate deter- 
mined for spherical enclosures will not 
guarantee that a minimum rise time for 
all enclosures will be obtained, but us- 
ing a minimum estimated rise time is con- 
servative, and so the approach is appro- 
priate for design of nonspherical 
enclosures. 



15 



An approximate relationship between the 
time from ignition to peak pressure 
and the enclosure volume is given by 
Zabetakis (12) as 



75 /V, msec. 



(1) 



where V is in cubic feet and t is in 
milliseconds. This is not a minimum rise 
time, but equation 1 can be used to scale 
a minimum rise time or maximum rise rate 
determined for one enclosure to enclo- 
sures with different volumes. A mini- 
mum rise rate for a 9-liter enclosure can 
be estimated from figure 1. Taking the 
tangent to the steepest part of the curve 
in figure 1,4 gives a pressure rise rate 
of 



MAX 



9 7 psi „ „ ./ 

1 n c = 9.2 psi/msec. 

10.5 msec 



(2) 



This same rate is given by the upper 
part of the curve in figure 15. Com- 
bining equations 1 and 2, the maximum 
rise rate for enclosures of other vol- 
umes is 



= 9.2 psi/msec 



75 V0.1352 ft3 



75 3/v"Tt3 



4.72 



psi/msec. 



Thus, the minimum rise time is 



(3) 



•"min 



MAX 



W 



4.72 



(4) 



where Pmax is in pounds per square inch, 
gage, and V is in cubic feet, 

or for a maximum explosion pressure of 
100 psig 



21 "*/¥, msec 



(5) 



where V is in cubic feet. 



Equation 5 is used unless the expected 
dynamic pressure is other than 100 psig. 



DESIGN FOR STATIC PRESSURE 



The sizes and shapes of enclosures can 
vary greatly, but basically the geometry 
of the sides, bottom, and cover can be 
categorized as rectangular, circular, and 
cylindrical. Approximate design proce- 
dures are given in the following para- 
graphs for treating each of these basic 
geometries. Some of these procedures 
include provisions for permanent defor- 
mations; others do not. If permanent 
deformations are included in the design, 
they must not exceed the limits set by 
MSHA. Use of these procedures will be 
illustrated by example problems in the 
design guide, but ultimately the designer 
must use his or her own judgment in the 
application of these methods. 

Rectangular Plates 

Solutions for rectangular plates under 
uniform static loads are taken from the 



work of Jones and Walters O ) . These 
solutions are based on rigid-perf ectly 
plastic theory and give the relation- 
ship between the applied pressure, P, 
and permanent normal deflections of the 
plate, Wq. For the geometry of fig- 
ure 2, equations 6 through 11 give the 
solutions for clamped and simply sup- 
ported plates. 




FIGURE 2. - Geometry of rectangular plates. 



16 



Clamped Plates 



P 1.236 w, 

— = 0.618 + 

P. h 



where P, 



3ay 



(3 - 2Co) 



4hi 

3 = a/b ^ 1, 
and ?o = e(/3 + 02 - 3). 



(1 + 3^) /TT^ - 3(2 + 3^) 

/3^rF 



(6) 

(7) 

(8) 
(9) 



Simply Supported Plates 



where 



3a, 



(3 - 2CJ 



2h- 



3h^ 



Pc 4 w2 I 5^+ (3 - 2Co)2 

T^ = 1 + 



(3 - Co) 



for j;- < 2": 



(10) 



(11) 



The solution for clamped plates has 
been shown by Jones and Walters (5^) to 
give good agreement with experimental 
results for plates with 3 in the range of 
1/3 to 1 and a/h in the range of 57 to 
161. Equations 6 and 10 both should be 
applied to conditions for which w^/h is 
less than 1/2. The current MSHA require- 
ment that the permanent deformation be 
less than 0.040 in/ft will usually give 
w^/h « 1/2. 

Boundary conditions for the walls and 
covers of typical enclosures are not well 
known. This occurs because some junc- 



tions between side walls are formed by 
bending the plate and some are produced 
by welding. Further, the strength of the 
weld joint will vary depending upon the 
weld joint design; so the designer must 
use his or her own judgment in defining 
the boundary conditions of the plate and, 
thus, the proper equations to be used. 
If there is uncertainty about the proper 
choice of boundary conditions, the 
assumption of simple support will give a 
conservative results. (The choice of 
boundary conditions will be further 
illustrated in the design guide by the 
use of worked examples.) 



17 



Circular Plates 

Solutions for circular plates are based 
on rigid-perf ectly plastic theory and de- 
fine the pressure for which a fully plas- 
tic collapse mechanism first develops in 
the plate. Lower bound collapse pres- 
sures for uniformly loaded plates of 
radius R and thickness h are given by 
Wood (11), and are as follows: 



Clamped Edges 
P, 



1.877 ayh2 



Simply Supported Edges 
P =^ 



(12) 



(13) 



where Oy is the yield stress of the mate- 
rial. Note that these equations are 
independent of displacement. They do not 
include the stiffening effects of mem- 
brane forces produced by large displace- 
ments and in-plane restraint at the 
boundaries. The stiffening effects of 
displacements have been accounted for in 
simply supported plates by Sawezuk (8). 
For plates with in-plane restraint at the 
boundary, he gives 

^ = l^l^^^^ (14) 

P 3 \h J 



center displacement of 
the plate and P^ is given by equation 13. 
As stated for rectangular plates, the 
permanent deformation now permitted by 
MSHA is so small that membrane effects 
are negligible. Thus equations 12 and 13 
are satisfactory for enclosure design. 



Also, in sizing the cover, it is recom- 
mended that the clamping effect of the 
bolts be neglected and that equation 13 
for simple support be used to determine 
cover thickness. 

Cylinders 

Cylinders designed for 150 psig will 
have thin walls, so that standard elastic 
formulas for thin wall cylinders can be 
used for design. Stresses will be uni- 
form across the wall thickness, t, and 
the design conditions can be based on the 
circumferential wall stress just reaching 
some prescribed design stress. For cyl- 
inders, a design stress just below the 
yield stress (95 pet ay) is chosen be- 
cause, once yielding occurs, the deforma- 
tions cannot be accurately predicted. 
Beyond the yield stress, the deformations 
will be controlled only by the effect of 
end restraints and the increase in 
strength of the material produced by 
strain hardening. Thus, the following 
pressure-thickness relationship is recom- 
mended for cylinders with a mean radius, 
R: 



Thin Wall Cylinder 



p ^ 0.95 Oyy ^ 



(15) 



This formula should be used with the min- 
imum yield stress of the material to 
assure that permanent deformations do not 
occur. Local yielding may occur at the 
ends of the cylinder or around penetra- 
tions, but the deformations will be well 
within the current MSHA requirement of 
0.040 in/ft. (The design of penetrations 
will be covered in another part of the 
guide. ) 



DESIGN FOR DYNA^aC PRESSURES 



Dynamic effects produced by transient 
loads can be important when the rise time 
of the loading is less than three times 
the fundamental period of the structural 



element. Methods for including dynamic 
effects in the design of enclosures for 
the explosion pressures are given in this 
section. 



18 



Elastic Behavior 

If the response Is elastic, that Is, 
no permanent deformations occur, the 
effect of the dynamic loads can be esti- 
mated from figure 3. This figure gives 
the dynamic effect in terms of a dynamic 
load factor (DLF). (DLF),^;^^ is the max- 
imum deflection produced by the dynam- 
ic loading divided by the deflection pro- 
duced by a static load of the same 
magnitude. 



figures contain formulas for the funda- 
mental frequencies of rectangular plates, 
circular plates, and cylinders. Note 
that for a cylinder, the fundamental mode 
corresponds to a uniform radial expansion 
and is not a bending mode. A uniform 
radial expansion is the type of response 
that a uniform internal pressure will 
produce in the cylinder. The next sec- 
tion also contains procedures for esti- 
mating fundamental frequencies of struc- 
tural coii5)onents. 



I 



To determine the DLF, one must know the 
rise time of the loading and the funda- 
mental frequency of the structural ele- 
ment. (The rise time of the explosive 
loading is determined from data given in 
"Design for Dynamic Pressures" section. ) 
In general, the shortest rise time will 
produce the greatest dynamic effect; how- 
ever dips in the DLF do occur at even 
multiples of t^/T,^. Because the rise 
times can vary, the minimum rise time 
should be estimated and the largest DLF 
for this rise time or any longer rise 
time should be chosen as the appropriate 
value. The fundamental frequency of the 
structural component can be calculated 
from equations in figures 4 and 5. These 



Once the DLF has been determined from 
figure 3, equations 6 through 15 can be 
used to calculate the required plate or 
wall thickness. Note that the design 
pressure is now the maximum explosion 
pressure as shown in the section 
"Design Pressures" multiplied by DLF. 
This procedure is approximate because 
the formulas for plates in the "Design 
for Static Pressure" section are based 
on some material plasticity and thus the 
response is not coii5)letely elastic. The 
error will be small. A method that 
accounts for material plasticity, and 
which can be used if larger permanent 
deformations are permitted, is given in 
the next section. 




FIGURE 3. = Maximum response of 1-degree elastic systems (undamped) subjected to constant force 
with finite rise time (9). 



19 





boundary'" 






wb /ph/D 


FOR VALUES 


OF b/a 




CONDITIONS 


0.4 


0.6667 


1.0 


1.5 


2.5 








11.45 

10.13 
12.13 
22.58 

23.65 


14.26 

10.67 

17.37 
23.02 

27.01 


19.74 

11.68 
28.95 
24.02 

35.99 


32.08 

13.71 
56.35 
26.73 

60.77 


71.56 

18.80 

145.50 

37.66 

147.80 










F 












V = 0.3 
/ / / / / / / / 








/ 
/ 
/ 
/ 
/ 




/ / / / / / / / / 

F 


' 






//////// 

V = 0.3 

/ / / / / / / / / 


^ 


' 




/ 
/ 
/ 
/ 




^ / /////// 





/^('^///^ 



KEY 



CLAMPED 



SIMPLE SUPPORT 



FREE 



D = 



Eh- 



12 (1- v) 



p = mass density 



FIGURE 4, = Fundamental circular frequencies for rectangular plates (derived from data in 
reference 3), 



20 



GEOMETRY 


CIRCULAR FREQUENCY 





10.22 ^ / D 


S.S. 



V = 0.3 


■ -f -v^ 


© 




p(l - V ) 
(breathing zone) 



Eh- 



12 (1- V ) 



mass density 



FIGURE 5. - Fundamental frequencies of circular plates and long cylinders (derived from data 
in reference 3)., 



Elastic-Plastic Behavior 

For elastic-plastic behavior, par- 
ticularly If larger permanent defor- 
mations are permitted, procedures devel- 
oped by Biggs (2^), Nemark (j6 ) , and Beck 
(_1_) are recommended. These methods are 
based upon the assumption of a defor- 
mation pattern for the structural ele- 
ment. The usual assumption for elastic 



behavior is the static deformed shape 
under the same distribution of loading. 
For plastic behavior, a hinge mechanism 
is postulated. Once the deflected shape 
has been chosen, the system (beam or 
plate) can be transformed into a one- 
degree-of -freedom (dof) system for which 
the dynamic response can be easily 
coiq)uted. 



21 



Based on the assumption of the static 
deformed shape and the formation of plas- 
tic hinges, figures 6 through 10 give 
transformation factors, resistance func- 
tions, spring constants, and shear reac- 
tions for uniformly loaded rectangular 
and circular plates. These quantities 
are given for different boundary condi- 
tions, plate aspect ratios, and material 
behavior. Definitions of these quanti- 
ties follow. 

Maximum Resistance . - This is the total 
load at which the plate will develop a 
fully plastic hinge mechanism. For the 
elastic solution of a clamped rectangular 
plate, it is the load at which a plastic 
hinge forms at the fixed boundary. For 
elastic-plastic and fully plastic behav- 
ior, it corresponds to the development of 
a hinge at the center as well as the 
fixed edge. The moments used to calcu- 
late the maximum resistance are 

M° = negative plastic bending 
psb moment capacity pev unit 
width at the center of 
edge b. 



psb 



total negative plastic 
bending moment capacity 
along edge b. 



pfb = total positive plastic 
bending moment along the 
midspan section parallel 
to edge b. 



M = same 
pf a J 

edge 



as above, but for 



and 



^pc ~ positive plastic bending 
moment capacity per unit 
length at the center of 
the circular plate, 

Mp2 = negative plastic bending 
moment capacity per unit 
length at the edge of 
the circular plate. 



Positive and negative moments need be 
considered only for concrete slabs in 
which these values may differ from each 
other. For homogeneous materials, there 
is no distinction between the positive 
and negative values. 

Spring Constant . - This is the spring 
constant for the plate expressed in terms 
of the elastic modulus, E, and the moment 
of inertia per unit width, Ig. It is 
calculated as the total load on the 
plate divided by the static center 
deflection. 

Dynamic Reaction . - The dynamic reac- 
tion is the shear at the boundaries ex- 
pressed as a fraction of the instanteous 
values of the applied load, F, and the 
resistance, R. To accurately determine 
the maximum shear, the time-history of 
the one-dof system must be calculated; 
however, because the loading from inter- 
nal explosions is idealized as a ramp 
function to a constant load, calculating 
the shear reaction at t,,, when the 
response is a maximum, and after the load 
has peaked, should give the maximum val- 
ue. An upper limit is found by taking 
the peak value of F and R to calculate 
the dynamic shear. 



Load Factor, K|^ , 



The load factor is 
the ratio of the load applied to the 
equivalent one-dof system to the total 
load applied to the plate. When the 
static deformed shape is assumed for the 
deformation pattern (as it is in figs. 
6-10), it is also the ratio of the spring 
constant of the equivalent system to the 
spring constant of the plate as given in 
the figures. 

Mass Factor, k^ . - This is the ratio of 
the mass of the equivalent one-dof system 
to the total mass of the plate. 



Load-Mass Factor, k|^n^ . - The 
factor is the ratio k^/k ^ . 



load-mass 



22 



Simple Suppo 



.y 



Strain 
Range 


a/b 


Load 
Factor 


Mass 
Factor 

•s, 


Load-Mass 

Factor 

^M 


Maximum 
Resistance 


Spring 
Constant 

k 


Dynamic Reactions 


\ 


\ 


Elastic 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.45 
0.47 
0.49 
0.51 
0.53 
0.55 


0.31 
0.33 
0.35 
0.37 
0.39 
0.41 


0.68 
0.77 
0.71 
0.73 
0.74 
0.75 


¥(Va-p.) 
i(l2.0M^^^ + 11.0M^J 

i(i2-° Va"^°-^ Vb) 

i(l2.0M^^^+ 9-«"pfb) 
i(^^-°Va- ^-^Vb) 


252 El^/a^ 
230 El^/a^ 
212 El^/a^ 
201 El^/a^ 
197 El^/a^ 
201 El^/a^ 


0.07 F + 0.18 R 
0.06 F + 0.16 R 
0.06 F + 0.14 R 
0.05 F + 0.13 R 
0.04 F + 0.11 R 
0.04 F + 0.09 R 


0.07 F + 0.18 R 
0.08 F + 0.20 R 
0.08 F + 0.22 R 
0.08 F + 0.24 R 
0.09 F + 0.26 R 
0.09 F + 0.28 R 


Plastic 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.33 
0.35 
0.37 
0.38 
0.40 
0.42 


0.17 
0.18 
0.20 
0.22 
0.23 
0.25 


0.51 
0.51 
0.54 
0.58 
0.58 
0.59 


^(Va-Pib) 
I (1^-° "pfa + ^1-° "pfb) 
i(l2-°"pfa + l°-3 Vb) 
i(l2.0«^^^. 5-8 Vb) 
i(l2.0M^^^. 9.3 Mp,,) 
i(l2.0M^^^. 9-°Vb) 










0.09 F + 0.16 R^ 

0.08 F + 0.15 R^ 

0.07 F + 0.13 R^ 

0.06 F + 0.12 R 
m 

0.05 F + 0.10 R 
m 

0.04 F + 0.08 R 


0.09 F + 0.16 R^ 
0.09 F + 0.18 R^ 
0.10 F + 0.20 R^ 
0.10 F + 0.22 R^ 
0.10 F + 0.25 R^ 
0.11 F + 0.27 R 



FIGURE 6. - Transformation factors for two-way slabs. Simple supports— four sides, uniform load; 
for Poisson's ratio = 0.3 (10). 



The purpose of the transformation fac- 
tors is to define a one-dof system that 
can be easily solved for its response to 
dynamic loads. The resulting equation is 



The fundamental period of the plate, 
T^, is also given by the one-dof system 
because the system is kinematically 
equivalent to the plate. The period is 
calculated as 



KmK-^ + KLkx = KlF (t). 



(16) 



where M, F, and k are the total mass, 
total load, and spring constant, respec- 
tively, of the plate, and K^ and K^^ are 
the transformation factors. Dividing 
equation 16 by Kl yields 



2Tr 



'Kk,M 



K.k 



= 2. /M. 



(18) 



Klu/^ + kx = F(t). 



(17) 



Equation 17 shows that only K^^^ is needed 
to obtain an equivalent one-dof system. 



Thus, only K^^, the spring constant from 
the figures, and the plate mass are 
needed to calculate T|^^ 

Numerical or closed-form solutions can 
be obtained easily for equation 17, but 
graphical solutions are also available. 
The solution for a ramp loading to a 



23 



'"•'^ 



Strain 
Range 


a/b 


Load 
Factor 

h 


Mass 
Factor 


Load-Mass 

Factor 

^M 


Maximum 
Resistance 


Spring 

Constant 

k 


Dynamic Reactions 


^.A 


^B 


Elastic 


1.0 
0.9 

0.8 
0.7 
0.6 
0.5 


0.39 
0.41 
0.44 
0.46 
0.48 
0.51 


0.26 
0.28 
0.30 
0.33 
0.35 
0.37 


0.67 
0.68 
0.68 
0.72 
0.73 
0.73 


^°-* ";sa 
— ;sa*^v 
— ;sa*^v 
'■^»;sa.-^Vb 
«-^";sa*--i^Vb 
-«;sa*^v. 


575 El^/a^ 
476 El^/a^ 
396 El^/a^ 
328 El^/a^ 
283 El^/a^ 
243 El^/a^ 


0.09 F + 0.16 R 
0.08 F + 0.14 R 
0.08 F + 0.12 R 
0.07 F + 0.11 R 
0.06 F + 0.09 R 
0.05 F + 0.08 R 


0.07 F + 0.18 R 
0.08 F + 0.20 R 
0.08 F + 0.22 R 
0.08 F + 0.24 R 
0.09 F + 0.26 R 
0.09 F + 0.28 R 


Elasto- 
Plastlc 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.46 
0.47 
0.49 
0.51 
0.53 
0.55 


0.31 
0.33 
0.35 
0.37 
0.37 
0.41 


0.67 
0.70 
0.71 
0.72 
0.70 
0.74 


i[^^-°(^fa-Va)*l^-°^fb] 
i[^^-°(Va*Va)^^^-°^fb] 
i[l^-°(Va^Va)*^°-^^fb 
i[^^-°(^fa*«psa)^ ^-^ Vb_ 

- 12. of M ^ + M 1 + 9.0 M ,, 
a [ \ pfa psa/ pfb 


271 Zlja^ 
248 EI /a" 
228 tlja- 
216 EI /a^ 
212 El^/a^ 
216 EI /a^ 


0.07 F + 0.18 R 
0.06 F * 0.16 R 
0.06 F ^ 0.14 R 
0.05 F + 0.13 R 
0.04 F ^ o.n R 
0.04 F + 0.09 R 


0.07 F + 0.18 R 
0.08 F + 0.20 R 
0.08 F + 0.22 R 
0.08 F + 0.24 R 
0.09 F + 0.26 R 
0.09 F + 0.28 R 


Plastic 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.33 
0.35 
0.37 
0.38 
0.40 
0.42 


0.18 
0.20 
0.22 
0.23 
0.25 


0.51 
0.51 
0.54 
0.58 
0.58 
0.59 


l[l^-°("pfa*"psa)*l^-°^fb] 

i[^2-°(Va^Va)^^°-^"pfb] 
i[^^°(Va^Va)* ^-'^fb] 
i[l^-°("pfa^Va)* '-^Vb] 
i[^^-°(Va^%sa)* '-"Vb] 










0.09 F + 0.16 R 
0.08 F + 0.15 R_^ 
O.OT F + O.n R^ 
0.06 F + 0.12 R 
0.05 F + 0.10 R 
0.04 F + 0.08 R 


0.09 F + 0.16 R 
0.09 F + 0.18 R_^ 
0.10 F + 0.20 R 
0.10 F + 0.22 R^ 
0.10 F + 0.25 R 
0.11 F + 0.27 R^ 



FIGURE 7. - Transformation factors for two-way slabs. Short edges fixed, long edges simply sup= 
ported; for Poisson's ratio = 0.3 (10). 



24 



Simple Support. 



^ Fixed 



Strain 
Range 


a/b 


Load 
Factor 


Mass 
Factor 


Load-Mass 
Factor 


Maximum 
Resistance 


Spring 
Constant 


Dynamic Reactions 


\ 


% 


Elastic 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.39 
0.40 
0.42 
0.43 
0.45 
0.45 


0.26 
0.28 
0.29 
0.31 
0.33 
0.34 


O.bl 
0.70 
0.69 
0.71 
0.73 
0.72 


^°-^ «;sb 

2° -2 «;sb 
"■2 «;sb 


575 EI /a^ 
600 El^/a^ 
610 El^/a^ 
662 El^/a^ 
731 El^/a^ 
850 El^/a^ 


0.07 F + 0.18 R 
0.06 F + 0.16 R 
0.06 F + 0.14 R 
0.05 F + 0.13 R 
0.04 F + 0'. 11 R 
0.04 F + 0.09 R 


0.09 F + 0.16 R 
0.10 F + 0.18 R 
0.11 F + 0.19 R 
0.11 F * 0.21 R 
0.12 F + 0.23 R 
0.12 F + 0.25 R 


Elasto- 
Plastic 


1.0 
0.9 
O.S 
0.7 
0.6 
0.5 


0.46 
0.47 
0.49 
0.51 
0.53 
0.55 


0.31 
0.33 
0.35 
0,37 
0.39 
0.41 


0.67 
0.70 
0.71 
0.73 
0.74 
0.74 


i[^^-°%fa^^^-°(Vb^Vb)] 
i[l2.0M^^^.10.3(Mp^^.M^^^)] 

i[l2.0M^^^. 9.b(mp^,.H^J] 

i[l=-°"pfa^ ^-^iVb^^fb)] 

- 12. OM^ + 9.0|M ^+M,J 
a [ pfa ^ psb pfb;_ 


271 El /a^ 
248 EI /a^ 
228 Elg/a^ 
216 El^/a^ 
212 EI /a^ 
216 El /a^ 


0.07 F + 0.18 R 
0.06 F + 0.16 R 
0.06 F + 0.14 R 
0.06 F + 0.13 R 
0.04 F + O.ll R 
0.04 F + 0.09 R 


0.07 F + 0.18 R 
0.08 F + 0.20 R 
0.08 F + 0.22 R 
0.08 F + 0.24 R 
0.09 F + 0.26 R 
0.09 F + 0.28 R 


Plastic 


1.0 
0.9 
0.8 
0.7 
0.6 
0.5 


0.33 
0.35 
0.37 
0.38 
0.40 
0.42 


0.17 

0.20 
0.22 
0.23 
0.25 


0.51 
0.51 
0.54 
0.58 
0.58 
0.59 


l[l^-° Va"^^-°(Vb"Vb) 
i[l^-°%fa^l^-°(Vb"Vb) 
i[l^-°%U*l°-^("psb*^4 

i[i^-°Va^ '•«(^sb*^4 

i[l^-°"pfa^ ^-^(Vb^^fb) 
1 [l2.0 M^^^ 4 9.0 (m^^^ . M^^^) 










0.09 F + 0.16 R__^ 
0.08 F + 0.15 R 
0.07 F + 0.13 R^ 
0.06 F + 0.12 R__^ 
0.05 F + 0.10 R 
0.04 F + 0.08 R_^ 


0.09 F + 0.16 R___ 
0.09 F + 0.18 R^ 
0.10 F + 0.20 R 
0.10 F + 0.22 R 
0.10 F + 0.25 R 
0.11 F + 0.27 R_^ 



FIGURE 8. - Transformation factors for two-way slabs. Short sides simply supported, long sides 
fixed; for Poisson's ratio = 0.3 (10). 



25 



w ^ / / ( ( ^ 



D 





















Dynamic 


Reactions 


Strain 




Load 
Factor 


Mass 
Factor 


Load-Mass 
Factor 




Maximum 




Spring 
Constant 










Range 


a/b 


\ 


\ 


Sm 




Resistance 




^ 


V 


^B 




1.0 


0.33 


0.21 


0.63 




"•^ ''psb 




810 El^/a^ 


0.10 F + 0.15 R 


0.10 F + 0.15 R 




0.9 


0.34 


0.23 


0.68 




"•* ";.b 




742 El^/a^ 


0.09 F + 0.14 R 


0.10 F + 0.17 R 




0.8 


0.36 


0.25 


0.69 




"•* "psb 




705 EI /a^ 


0.08 F + 0.12 R 


0.11 F + 0.19 R 


Elastic 






















0.7 


0.38 


0.27 


0.71 




2'- 2 «;sb 




692 El^/a^ 


0.07 F + 0.11 R 


0.11 F + 0.21 R 




0.6 


0.41 


0.29 


0.71 




"•^%sb 




724 EI /a^ 


0.06 F +-0.09 R 


0.12 F + 0.23 R 




0.5 


0.43 


0.31 


0.72 




^°-2 "psb 




806 El^/a^ 


0.05 F + 0.08 R 


0.12 F + 0.25 R 




1.0 


0.46 


0.31 


0.67 


l['^-°(Va^ 


Va)*l^-°(Vb^ 


^j] 


252 El^/a^ 


0.07 F * 0.18 R 


0.07 F + 0.18 R 




0.9 


0.47 


0.33 


0.70 


i[^^-°(Va^ 


Va)*^^-°("pfb" 


^sjl 


230 EI 1^ 


0.06 F + 0.16 R 


0.06 F + 0.20 R 


Elasco- 


0.8 


0.49 


0.35 


0.71 


l[l^-°(Va^ 


Va)*^°-^(Vb* 


%=^)j 


212 El^/a^ 


0.06 F + 0.14 R 


0.06 F + 0.22 R 


Plastic 












\ / 


\' 










0.7 


0.51 


0.37 


0.73 


i[l^-°(Va^ 


Va)^ '-nvb^ 


",..) 


201 El^/a^ 


0.05 F + 0.13 R 


0.06 F + 0.24 R 




0.6 


0.53 


0.39 


0.74 


ih°(Va^ 


Va)^'-KVb^ 


V.)] 


197 El^/a^ 


0.04 F + 0.11 R 


0.09 F + 0.26 R 




0.5 


0.55 


0.41 


0.75 


l[l^-°(^ta- 


Va)* '-"(Vb^ 


V.)] 


201 EI /a^ 


0.04 F + 0.09 R 


0.09 F + 0.28 R 




1.0 


0.33 


0.17 


0.51 


\ 12.0 ^Mpj^ + 


"„.)•"■» (v.- 


vj) 





0.09 F + 0.16 R 


0.09 F + 0.16 R_^ 




0.9 


0.35 


0.18 


0.51 


l[^^-°(Va^ 


"psa)^ll-°(Vb^ 


"»0] 





0.08 F + 0.15 R 


0.09 F + 0.18 R 


Plastic 


0.8 


0.37 


0.20 


0.54 


i[i^-°(Va^ 


Va)^^°-^(Vb^ 


"..) 





0.07 F + 0.13 R 


0.10 F + 0.20 R 




0.7 


0.38 


0.22 


0.58 


\ 12.0f Mpj^ + 


Va)^ '■«(Vb* 


%.\ 





0.06 F - 0.12 R_. 


0.10 F + 0.22 R__^ 




0.6 


0.40 


0.23 


0.58 


i[l^°(»pfa^ 


«psa)* '-^("pfb^ 


V.)] 





0.05 F + 0.10 R^ 


0.10 F + 0.25 R^ 




0.5 


0.42 


0.25 


0.59 


^ 12.0 (m j^ + 


Va)* '"("pfb- 


".«)] 





0.04 F + 0.08 R 


0.11 F + 0.27 R_^ 



FIGURE 9. - Transformation factors for two-way slabs. Fixed supports, uniform load; for Poisson's 

ratio = 0,3 (10). 



26 





Fixed Edges 



Simple Supports 



Edge 
Condition 


Strain 
Range 


Load 
Factor 


Mass 
Factor 

•Si 


Load-Mass 
Factor 


Maximum 
Resistance 


Spring 
Constant 


Dynamic 
Reaction 


Simple 
Supports 


Elastic 


0.46 


0.30 


0.65 


18.8 M 

pc 


216 El/a^ 


0.28 F + 0.72 R 


Plastic 


0.33 


0.17 


0.52 


18.8 M 

pc 





0.36 F + 0.64 R 


Fixed 
Supports 


Elastic 


0.33 


0.20 


0.61 


"•1 V 


880 El/a^ 


0.40 F + 0.60 R 


Elasto- 
Plastic 


0.46 


0.30 


0.65 


i«-«(v"v) 


216 El/a^ 


0,28 F + 0.72 R 


Plastic 


0.33 


0.17 


0.52 


i«-«(v"v) 





0.36 F + 0.64 R 
m 



FIGURE 10, = Transformation factors for circular slabs, Poisson's ratio = 0.3 (10). 



constant value is given by figure 11. 
Figure 11^5 gives the maximum displace- 
ment, x^, relative to the displacement at 
yielding, Xg, and figure ll5 gives the 
time, tj^, at which the maximum response 
is reached relative to the rise time of 
the load, t,.. Both of these quantities 
are given as functions of the maximum 
resistance, R^, the maximum total applied 
load, F, and the fundamental period of 
the plate, T,^, To use figure 3, one must 
determine the following quantities: 

Tjg - calculated by equation 18, 



the rise time of the load, 

the maximum value of the load on 
the plate which is the product of 
the maximum value of the pressure 
and the plate area. 



R^ - maximum resistance from figures 6 
through 10. 

Xg - deflection at which the plate 
yields. It is calculated as the 
maximum resistance for elastic be- 
havior divided by the spring con- 
stant from figures 6 through 10. 

Note that to use figure 4, the only 
transformation factor that is needed is 
Klm, used in equation 18 to calculate Tf^. 
Also, it is necessary to know in advance 
how the structure will respond; that 
is, will the structure remain elastic, 
experience mild plasticity, or undergo 
gross deformations? This determines 
the choice of parameters from the fig- 
ures. If elastic behavior is expected, 
choose the parameters that correspond to 
elastic behavior; if gross plasticity is 



27 




r t, \£L:\2i 



Jr 

Load 



Resistance 
function 



Displacement 
function 




.4 .6.81.0 2 

tr/tn 



4 6 810 20 



Load Resistance Displacement 

function function . 

■ ■ ' I L_L 



.4 .6.81.0 2 

tr/tn 



4 6 810 20 



FIGURE 11, - Maximum response of undamped single-degree-of=freedom elastic=plastic syster 
to step pulse with finite rise time (TO). 



expected, choose the parameters for 
plastic behavior; if mild plasticity is 
expected, average the values for elastic 
and plastic behavior. Another alterna- 
tive is to solve equation 17 for the 
response of the structural element. 
When solving equation 17, note that the 
parameters that define the one-dof 
approximation mist change as the system 
yields. 



As explained previously, the maximum 
response is usually produced by a loading 
with the shortest rise time; however, 
dips in the response do occur at even 
multiples of the t^/T,^ ratio. Therefore, 
it is recommended that the response be 
taken as the maximum value that occurs 
for all rise times equal to or greater 
than the minimum rise time as determined 
in the "Design Pressures" section. 



CONCLUSIONS 



The authors hope that the design guide 
that is being developed by the Federal 
Bureau of Mines will be of great benefit 
to the manufacturers of explosion-proof 
enclosures. This can only happen if the 
guide contains the appropriate informa- 
tion in a suitable format and if the 
guide is accepted and used by the indus- 
try. Although this paper does not show 



the final format of the guide, it does 
give an overview of its contents. The 
format will be similar to that used in 
this paper, but it will be con?)lemented 
by charts in some areas to make the solu- 
tions simpler. Comments on the scope, 
contents, and format of the guide are 
solicited by the authors in order to 
tailor the guide to the mining industry. 



28 



BIBLIOGRAPHY 



1. Beck, J. E., B. M. Beaver, H. S. 
Levine, and E. Q, Richardson. Single- 
Degree-of-Freedom Evaluation. Air Force 
Weapons Lab Rept. AFWL-TR-80-99, March 
1981, 306 pp.; available from Kirtland 
Air Force Base, N. Mex. 

2. Biggs, J. M. Simplified Analysis 
and Design for Dynamic Load. Ch. 7 in 
Structural Design for Dynamic Loads. 
McGraw-Hill Book Co., Inc., New York, 
1959, 453 pp. 



Urbana, 111., Tech. Rept. on Off. Naval 
Res. Contract N60-RI-71, December 1950, 
pp. 34-44. 

7. Perlee, H. E., F. N. Fuller, and 
C. H. Saul. Constant -Volume Flame Propa- 
gation. BuMines RI 7839, 1974, 24 pp. 

8. Sawezuk, A. Large Deflections of 
Rigid-Plastic Plates. Proc. 11th Inter- 
nat. Cong. Appl. Mech. , Munich, Germany. 
Sprenger-Verlag, 1964, pp. 224-228. 



3. Blevins, R. D. Formulas for Natu- 
ral Frequency and Mode Shape. Van 
Nostrand Reinhold Co. , New York, 1979, 
492 pp. 



9. Scott, L. W. Some Design Factors 
for Windows and Lenses Used in Explosion- 
Proof Enclosures. BuMines IC 8880, 1982, 
9 pp. 



4. Francis, P. H. , and J. Lankford. 
Recommended Acceptance Testing Criteria 
for Adhesives and Sealants for Explosion- 
Proof Electrical Enclosures (Contract 
H0387009, Southwest Res. Inst.). BuMines 
OFR 129-80, Jan. 9, 1980, 67 pp.; NTIS 
PB81-128738. 

5. Jones, N. , and R. M. Walters. 
Large Deflections of Rectangular Plates. 
J. of Ship Res., June 1971, pp. 164-171. 

6. Newark, N. M. Methods of Analy- 
sis for Structures Subjected to Dynam- 
ic Loading. University of Illinois, 



10. U.S. Army Corps of Engineers. 
Manual EM 1110-345-415, Design of Struc- 
tures To Resist the Effects of Atomic 
Weapons. 1975, 136 pp.; available from 
Defense Documentation Center, Logistic 
Agency, Cameron Station, Alexandria, Va. 

11. Wood, R. H. Plastic and Elastic 
Design of Slabs and Plates. The Ronald 
Press Co., New York, 1961, 344 pp. 

12. Zabetakis, M. G. Flammability 
Characteristics of Combustible Gases and 
Vapors. BuMines B 627, 1965, 121 pp. 



29 



HIGH-VOLTAGE, EXPLOSION-PROOF LOAD CENTERS 
By George Conroy,'' Randy Berry, 2 and Robert Gillenwater2 



ABSTRACT 



To attain future underground mine pro- 
duction considered to be necessary by the 
Department of Energy, voltages higher 
than the presently permitted 4,160 V must 
be carried to the close vicinity of the 
mining machines; otherwise, trailing 
cable size and weight creates handling 
problems. A project has been running for 



the past year to delineate approval tests 
and acceptance criteria for explosion- 
proof load centers that will permit oper- 
ation inby the last open crosscut. The 
investigation includes fabrication and 
testing of a load center. Tentative 
specifications are presented in this 
report. 



INTRODUCTION 



Present methods of developing some un- 
derground coal mines and some extensive 
noncoal mines require the transfer of 
large amounts of electric power through 
long lengths of trailing cable. A prac- 
tical alternative to specifying very 
large diameter cables, used to reduce 
voltage drop, is to transfer the power at 
high voltage and relatively low current 
levels. The voltage might then be trans- 
formed to a working level by equipment 
located near the mining machine. In cer- 
tain circumstances, either inherent in 
the mode of operation or because uncon- 
trollable methane "bursts" can occur, the 
stepdown transformer and associated 
switching apparatus may become surrounded 
by methane-air atmosphere, leading to the 
possibility of a mine explosion if the 
equipment has not been constructed to be 
permissible. 

Verifying permissibility is the respon- 
sibility of Mine Safety and Health Admin- 
istration's (MSHA) Approval and Certifi- 
cation Center in Triadelphia, W. Va. The 

^Supervisory electrical engineer (re- 
tired) , Pittsburgh Research Center, Bu- 
reau of Mines, Pittsburgh, Pa. 

^Technical staff consultant, Foster- 
Miller Associates, Waltham, Mass. 

^Senior engineer, Foster-Miller Associ- 
ates, Waltham, Mass. 



most important guidelines for the deter- 
mination are the set of regulations con- 
tained in Title 30, Code of Federal Regu- 
lations (CFR), Part 18. Part 18 gives 
very little guidance for test and accept- 
ance of equipment to be energized at 
voltages from 1,000 to 4,160 V, and does 
not even consider voltages above 4,160 V. 
In an effort to facilitate the use of 
higher voltages, the Department of Energy 
(DOE) funded a Federal Bureau of Mines 
project to develop acceptance tests and 
criteria that MSHA could use to supple- 
ment current Part 18 regulations. At the 
same time, DOE has funded MSHA to develop 
an explosion test gallery capable of 
accommodating and testing the large en- 
closures necessary for high-voltage 
transformers and switchgear. At MSHA's 
option, this gallery could be transported 
to a high-power test facility to conduct 
explosion tests on load centers using a 
high-voltage source ranging up to the 15- 
kV maximum now contemplated. These tests 
would measure the internal pressures and 
temperatures resulting from the occur- 
rence of an arcing electrical fault in 
the presence of an explosive methane-air 
mixture. At other times, the MSHA gal- 
lery would be utilized at Triadelphia, 
W. Va. , for explosion testing as pres- 
ently conducted on all permissible 
equipment. 



30 



This paper examines the following spe- 
cial considerations concerning high volt- 
age load centers: 

• Power rating: maximum levels 

• Maximum current to an arcing fault 

• Maximum arc duration 



• Electrical clearances 

• Insulating materials 

Each of these five topics is dis- 
cussed separately in the following 
sections. 



SPECIAL CONSIDERATIONS CONCERNING APPROVAL AND TESTING CRITERIA FOR 
HIGH-VOLTAGE PERMISSIBLE LOAD CENTERS 



Power Rating of Load Centers 

Maximum power requirements for under- 
ground coal mine section load centers, 
whether or not permissible, are estimated 
to be within the 2,000-kVA upper limit 
addressed in the ongoing Bureau program. 
One limiting factor is the physical size 
of such equipment as con^sared with the 
dimensions of existing mine entries. 
Load centers capable of delivering more 
than 2,000 kVA are likely to pose major 
mobility problems in underground coal 
mines. This may be less of a problem in 
such noncoal settings as oil shale mines. 
However, the question of permissibility 
has not yet been resolved -in that appli- 
cation, nor have particular equipment 
rating and size requirements been 
specified. 

Another limiting factor for lower volt- 
age permissible load centers is cable 
size. Present Part 18 requirements for 
three-conductor trailing cable for use at 
voltages up to 5 kV specify a maximum 
ampacity of 305 A for 350 MCM shielded 
cable. This places an upper limit on the 
power rating of permissible load centers, 
assuming power input through a single 
trailing cable, as shown in figure 1. As 
indicated by the curve, a 2,000-kVA load 
center could be supplied at any line 
voltage level above 3.8 kV. No more than 
500 kVA could be utilized in a load cen- 
ter designed for 1.0-kV operation. How- 
ever, a 350 MCM cable is much larger than 
most mines would care to use for supply- 
ing power to individual sections. Sup- 
posing the use of a 4/0 cable, with a 
maximum ampacity of 220 A, yields the 
lower curve shown in figure 1. Using 



this cable, a 2,000-kVA permissible load 
center could be supplied at any line 
voltage level above 5.1 kV. A 500 kVA 
load center would require a minimum oper- 
ating voltage of 1.3 kV. Adoption of 
cable rating tables that allow a higher 
conductor temperature would result in 
higher current levels and correspondingly 
higher allowable kilovoltampere ratings. 
This is not likely to occur without 



2,000 


III/ 
350 MCM / 
cable-y / 


M/'l 1 1 1 1 1 1 1 


1 


1,000 


- // 




- 


800 


- 1 




- 


600 


- II 




- 


400 


11 




- 




/ / cable 




- 


200 


"/ 




" 


100 


1 




_ 


80 






- 


60 






- 


40 






- 






Present 30 CFR 18 voltage 
limitation 4.16 kV 


- 


20 


- 




- 


10 


1 1 1 1 


1 1 1 1 1 1 1 1 1 


1 



I 2 3 4 5 6 7 8 9 10 II 12 13 14 15 
LINE VOLTAGE, kV 

FIGURE 1, - Regions of practical interest, high- 
voltage permissible load centers. 



31 



substantial justification. Therefore, 
the practical range of interest in pre- 
paring special acceptance criteria for 
very high voltage peinnissible load cen- 
ters includes input powers from 500 to 
2,000 kVA, and line voltages from 1.0 kV 
to a maximum of 15 kV, the highest volt- 
age expected to be used underground in 
the forseeable future. Although load 
centers operating at less than 4.16 kV 
could supposedly be examined under the 
existing regulations, a dearth of prece- 
dents in the region above 1 kV justifies 
interest with regard to new test and 
acceptance criteria. 

Maximum Current to an Arcing Fault 

One of the principal questions regard- 
ing permissible load centers is whether 
or not their explosion-proof charac- 
teristic can be compromised if the energy 
released by an arcing electrical fault is 
added to that generated by a simultaneous 
methane-air explosion within the enclo- 
sure. The energy contributed by the arc- 
ing fault depends on the fault current. 
Because the arc has a finite resistance, 
this fault current will be less than the 
current possible in a "bolted" (zero 
resistance) short circuit. Thus, the 
worst case situation can be conserva- 
tively considered as the occurrence of an 
arcing fault with a current flow equal to 
the maximum available current through a 
bolted fault at the internal terminations 
of the trailing cable. The shortest 
length of trailing cable connecting the 
load center to the substation supplying 
its power would usually be 500 to 1,000 
ft, while the maximum might be 21,000 ft. 
The cable impedance at the shortest dis- 
tance is so small as to be a negligible 
factor in limiting the fault current, so 
that substation transformer impedance 
must provide the worst case limit. At 
the longest distance, trailing cable 
impedance along with the arc voltage 
might limit fault current to a rather low 
value, thus creating a sensitive situa- 
tion with regard to the ability of 



primary circuit protection devices to 
interrupt the current rapidly enough to 
prevent a serious buildup of pressure in 
the enclosure. This problem is explored 
in more detail in the next subsection. 

Continuing the conservative approach, 
the substation is assumed to be connected 
to an "infinite bus" at the point of 
supply from the utility. This substation 
is presumed to be dedicated to the single 
load center concerned so that its trans- 
former impedance may be related directly 
to this circuit. This impedance is 
assumed to be 5 pet of the impedance that 
would yield rated load current, and 
therefore rated apparent power, for any 
given line voltage. Taking some discrete 
values of line voltage and power rating, 
transformer phase-to-phase impedance 
would appear as shown in table 1. This 
value can be used to calculate the 
current available to a bolted fault 
(neglecting cable impedance). The im- 
pedance per thousand feet of the smallest 
size cable that could be used for the 
selected values, per Part 18, is also 
listed in table 1. This value will be 
used for estimating the current limiting 
effect of long lengths of cable. 

Table 2 and figure 2 show the current 
available to ^bolted faults as calculated 
using the preceding assumptions. The 
largest available fault current is 8,175 
A in the 1-kV, 500-kVA unit. The availa- 
ble current in the 4.16-kV, 2,000-kVA 
unit is only slightly less. A worst-case 
test as suggested by the analysis is to 
be one with 10,000 A available from a 
4.16-kV source, supplying an arcing fault 
in a 2,000-kVA load center. As the 
intent is to verify only that the enclo- 
sure can withstand any developed internal 
pressures and temperatures, actual test- 
ing would be performed using separately 
introduced test electrodes rather than 
actual component members. An arbitrary 
electrode spacing of 6 in is recommended, 
with the arc being started by means of a 
connecting filament. 



32 



TABLE 1. - Primary cable values 



Line voltage, kV 



Power 

rating, 

kVA 



Line current at 

rated power, 

A 



Transformer 

5 pet 

impedance, 

ohms 



Mine cable 
size 



Cable 

impedance, 

ohms per 

1,000 ft 



1.0. 



2.3. 



4.16. 



7.2, 



12.5, 



15. 



500 

500 
1,000 

1,000 
1,500 
2,000 

1,000 
1,500 
2,000 

1,000 
1,500 
2,000 

1,000 
1,500 
2,000 



289 

126 
251 

139 
208 
278 

80 
120 
160 

46 
69 
92 

38 
58 
77 



0.173 

.916 
.458 

1.499 
.999 
.749 

4.490 
2.993 
2.245 

13.351 
9.021 
6.794 

19.485 

12.990 

9.742 



350 MCM 

AWG 2 
300 MCM 

AWG 1/0 
AWG 4/0 
350 MCM 



AWG 
AWG 



AWG 2/0 



AWG 
AWG 
AWG 



AWG 6 
AWG 6 
AWG 4 



^Smallest size permitted by 30 CFR 18. 

TABLE 2. - Peak available current to a bolted fault 



0.116 

.386 
.125 

.255 
.151 
.116 

.296 
.153 
.102 

.468 
.296 
.237 

.468 
.468 
.296 




33 



1 \ — I — \ — I \ \ r 



I I I 




I I I I I I I I I I I I I I 



i 2 3 4 5 6 7 8 9 10 II 12 13 14 15 
LINE VOLTAGE, kV 

FIGURE 2. - Peak available current for various 
power ratings. 

Maximum Arc Duration 

Modern Industrial practice with regard 
to power systems In the 1- to 15-k.V range 
calls for the use of circuit protection 
devices capable of Interrupting the cir- 
cuit within 50 msec (3 cycles at 60 Hz). 
Circuit Interruptions In this short 
Interval would occur after a fault to 
ground activates a sensing relay, which 
signals the circuit Interrupter to Initi- 
ate action. The total Interrupting time 
on a ground fault might therefore be as 
much as 83 msec (5 cycles). If, for any 
reason, the ground fault relay should 
fall to act. It Is Important that circuit 
overcurrent protection can act to Inter- 
rupt the current; otherwise, an arc of 
long duration and extreme destructlveness 
could result. The question therefore 
arises as to whether or not the combined 
effect of transformer Impedance, Im- 
pedance of the longest trailing cable 
likely to be used, and arc voltage drop 
can limit the circuit current to a value 
that Is too low to permit reliable set- 
ting of overcurrent relays without nui- 
sance tripping on motor startup. 

Arc voltage Is known to be a decreasing 
function of arc current, becoming fairly 



constant for currents lower than about 
300 A. Arc voltage then slowly Increases 
with Increasing current, probably never 
exceeding 1,000 V under the conditions 
described. Some additional Investigation 
Is justified In this area. If an arc 
voltage of 1,000 V Is assumed (except In 
the case of the 1.0-kV load center), this 
value can be used together with the Im- 
pedances listed In table 1 to calculate 
the ratio of minimum overcurrent to rated 
current for various line voltages and 
power ratings. These ratios are listed 
In table 3. The ratio at 1.0 kV line 
voltage Is uncertain, as 1,000-V arc 
voltage obviously cannot be assumed. 
However, this value of line voltage Is 
already covered by 30 CFR 18, and no 
Instances of long-continued arcing have 
been reported. 

From table 3, the smallest calculated 
Ip/Ip ratio Is 3.5, where Ip Is the fault 
current and Ip^ Is the rated current. 
This occurs for a 2.3-kV, 500-kVA load 
center. In order for this value to be 
reached by a motor startup 
motor larger than 425-hp must 
directly online, under load, 
starting current that Is 5.5 
current. The more common starting cur- 
rents for mine equipment are about 2.5 
times rated current. Motors larger than 
AOO-hp powered from a 2.3-kV line can 
certainly be equipped with step starters. 
If necessary, to avoid nuisance Inter- 
ruptions even when started under full 
load. Therefore, the overcurrent protec- 
tion for the load center primary circuit 
can be set below 3.5 times rated current, 
with only a very small time delay — less 
than 2 cycles — to allow for unavoidable 
line transients. 

From this analysis, circuit protective 
devices can be presumed capable of Inter- 
rupting an arcing fault, either by ground 
fault relaying or by the circuit over- 
current relaying, within 7 cycles. This 
permits calculation of the maximum energy 
In the fault as shown In the last column 
of table 3. The free volume within the 
enclosure should be sufficient to avoid 
excessive buildup of pressure when this 
energy Is combined with that released by 
a methane explosion. 



current, a 
be started 
to give a 

times rated 



34 



TABLE 3. - Arc currents and energies at various line voltages 



Line voltage, 


Power 


Calculated arc 


Ratio, 


Maximum fault 


kV 


rating, 
kVA 


current, A 


If/Ir 


energy, kJ 


1.0 


500 


(0 


(0 


(O 


2.3 


500 


440 


3.5 


118 




1,000 


1,162 


4.6 


312 


4.16 


1,000 
1,500 


992 

790 


7.1 
3.8 


481 




383 




2,000 


1,600 


5.8 


776 


7,2 


1,000 
1,500 


577 
996 


7.2 
8.3 


485 




837 




2,000 


1,409 


8.8 


1,184 


12.5 


1,000 


491 


10.7 


715 




1,500 


753 


10.9 


1,098 




2,000 


975 


10.6 


1,422 


15 


1,000 
1,500 


476 
612 


12.5 
10.5 


833 




1,071 




2,000 


875 


11.3 


1,531 



lvalue uncertain, 1,000-V arc voltage cannot be assumed 



Electrical Clearances 

A special laboratoiry investigation^ was 
conducted to determine the effect of a 
methane-air explosion in facilitating the 
occurrence of an arc between electrodes 
sustaining a high potential difference. 
The mixture at which arcing is most 
likely to occur is 9.8 pet methane, the 
same mixture that gives maximum pressure 
and highest flame temperature. Using 
this mixture, the critical voltages for 
arc initiation were determined for a 
number of gap distances. A linear 
relationship was found to exist between 
electrode spacing and the minimum initia- 
tion voltage under explosion conditions. 
As presented in figure 3, curve B indi- 
cates that arcing can occur during the 
explosions at a gap distance more than 18 
times as great for a given voltage as the 
spacing for an arc in air, shown by curve 
A. For design purposes, a factor of 1.5 
times the curve B spacing is considered 

■^Scott, L. W. / and Joseph G. Dolgos. 
Electrical Arcing at High Voltage During 
Methane-Air Explosions Inside Explosion- 
Proof Enclosures. BuMines TPR 115, 1982, 
9 pp. 



to be the minimum recommended clearance 
between energized members. 

Insulating Materials Used in 
Explosion-Proof Enclosures 

It has long been known that certain 
organic insulating materials, when 
decomposed by the action of an electrical 
arc, can liberate large quantities of 
hazardous gases. This has never been a 
problem in the United States for per- 
missible equipment operating at less than 
incidents of enclosure 
this phenomenon have 
Canadian and European 
use of high-voltage 
equipment in explosion-proof enclosures 
is commonplace. 4 



1,000 V. However, 
failure caused by 
been reported' in 
mines where the 



^Barbero, 



L. P. 



E. H. Davis, 



and 



H. Lord. Hazards Resulting From the 
Volatilization by Electric Arcing of 
Insulating Materials in Flameproof Equip- 
ment. Pres. at 15th Internat. Conf. on 
the Safety in Mines Research, Karlovy, 
Vary, Czechoslovakia, Sept., 18-21, 1973, 
8 pp.; available for consultation at 
Bureau of Mines Pittsburgh Research Cen- 
ter, Pittsburgh, Pa. 



35 




z^ 



Curve A 



-Curve B 



FIGURE 3. 



10 



20 



30 



40 



70 



80 



90 



100 no 



50 60 
SPACING, mm 

Minimum arc of voltages versus air-gap spacings of electrodes. Curve A, in air; Curve B, 
in 9.8 pet methane-air mixtures. 



Owing to the widespread use of organic 
plastic insulating materials in the elec- 
trical industry, a conplete ban of such 
materials in high-voltage explosion-proof 
enclosures is not practical. However, 
the use of such insulators must be accom- 
panied by special efforts that minimize 
the amount of such material used and 
insure that the materials that are used 



have been tested and found highly resist- 
ant to the destructive effects of elec- 
trical arcs. In addition, protective 
devices inside a permissible enclosure 
can be used to detect arcing, overpres- 
sure, or excessive temperatures and dis- 
connect incoming power before a hazardous 
condition develops. 



CONCLUSIONS 



The preceding discussion describes 
some of the problems and reasoning in- 
volved in the development of acceptance 
criteria for high voltage permissible 
load centers. The product of this devel- 
opment to date is the document attached 
as appendix A. This document has been 



reviewed by various government and indus- 
try personnel and comments received up to 
now have been incorporated. Further 
views and comments are invited as the 
opportunity for revision will continue 
for some time. 



36 



APPENDIX A. —RECOMMENDED APPROVAL AND TESTING CRITERIA 
FOR HIGH-VOLTAGE PERMISSIBLE LOAD CENTERS! 

INTRODUCTION 



The objective of phase I of this pro- 
gram is the development of criteria for 
use by the Mine Safety and Health Admin- 
istration (MSHA) in testing and approving 
permissible load centers with maximum 
voltage ratings of 15 kV and maximum 
power capacities of 2,000 kVA. A draft 
of the recommended criteria follows this 
brief introduction. 

The distinguishing feature between the 
existing requirements of Title 30, Code 
of Federal Regulations (CFR), Part 18, 
and the requirements of 30 CFR 18 as sup- 
plemented by these criteria, is that the 
latter permits approval of permissible 
equipment consisting, in full or in part, 
of high-voltage circuits and con5)onents. 
The criteria address only those factors 
that are influenced by the higher volt- 
age. These factors are discussed in num- 
bered paragraphs that follow. 

The organization of the recommended 
criteria follows closely that of Part 18. 
This organization has the obvious advan- 
tage of familiarity and is the natural 
result of "extending" Part 18 to cover 
the type of equipment of interest in this 
program. 

For ease of reference, selected sec- 
tions of Parts 18 and 75 are reproduced 



in appendix B. These sections are appli- 
cable to permissible load centers with 
one recommended change. That change is 
to Section 18.47 — Voltage Limitation. It 
is recommended that the 4,160-V limita- 
tion found in Section 18.47(d) be de- 
leted. Recommended changes to the texts 
of Sections 18.47(d), 18.47(d) (3), and 
18.47(d)(5) can be found in the comment 
following paragraph 14. 

With the 4,160-V limitation removed. 
Part 18 will continue to be the sole 
source of requirements for approval of 
low- and medium-voltage permissible elec- 
trical equipment. However, Section 
18.47(d)(6) reserves for MSHA the right 
to require additional safeguards for 
high-voltage equipment. These criteria 
represent those additional safeguards re- 
quired for the approval of load centers, 
transformers, switchgear, and related 
equipment with maximum voltage ratings of 
15 kV and maximum power capacities of 
2,000 kVA. Similar criteria should be 
developed for other high-voltage applica- 
tions, as needed. 

Comments and suggestions from interest- 
ed persons are welcomed. Written com- 
ments are especially appreciated. 



PART A—GENERAL PROVISIONS 



1. Purpose 

The purpose of these criteria is to 
specify the design and testing require- 
ments to be used by MSHA in approving 
load centers, transformers, and switch- 
gear as permissible for use in gassy 
mines or tunnels. These criteria are a 

^This work was performed under Bureau 
of Mines contract H0308093. 



supplement to the existing requirements 
of 30 CFR 18, all of which apply unless 
specifically modified or replaced by 
parts of these criteria. These pro- 
visions are applicable to load cen- 
ters, transformers, switchgear, and 
related equipment operating at maximum 
voltages of 15 kV. For transformers, 
the maximum secondary voltage is 4.16 
kV, and the maximum power rating is 
2,000 kVA. 



37 



2, Definitions 

a. Corona (partial discharge). A 
type of localized discharge resulting 
from the ionization of gas in an insula- 
tion system when the voltage stress ex- 
ceeds a critical value. The ionization 
is localized over only a portion of the 
distance between the electrodes of the 
system. 

b. Corona inception voltage. The 
lowest voltage at which corona occurs 
as the applied voltage is gradually 
increased. 

c. Corona extinction voltage. The 
highest voltage at which corona no longer 
occurs as the applied voltage is grad- 
ually decreased from above the corona 
inception voltage. 

d. Phase segregation. The isolation 
of each phase conductor of an electrical 
circuit by means of a conqjletely sur- 
rounding grounding metallic covering or 
enclosure. 

3. Quality Assurance 

The factory inspection form required 
by 30 CFR 18.6(k) shall specify a system- 
atic checking sequence designed to assure 
the quality of each load center. The 
form shall include, but not be limited 



to, a detailed checklist in the following 
areas: 

• Explosion-proof construction — with 
special attention paid to flange gap 
dimensions, surface finishes, cable 
entrances, plugs and receptacles, joints, 
covers, fasteners, and welding quality. 

• Components — a visual inspection 
for damaged or faulty components prior to 
assembly. 

• Assemblies — insure that all conpo- 
nents, subassemblies, and assemblies are 
fitted in accordance with appropriate 
drawings and specifications. 

• Operation — check for proper opera- 
tion of all mechanical devices and link- 
ages. Also check for proper installation 
and operation of pressure relief, venti- 
lation and drainage devices, pressure 
rise detectors, over-temperature sensors, 
and other protective devices. 

• Ancillaries — ^where applicable, all 
preceding checks will be carried out on 
ancillary components, assemblies, and 
enclosures. 

Emphasis should be placed on systematic 
organization of the inspection form to 
insure that all checks are made at appro- 
priate times. 



PART B~CONSTRUCTION AND DESIGN REQUIREMENTS 



4. Limitation of External Surface 
Temperatures 

The temperature of the external sur- 
faces of mechanical or electrical load 
center conq)onents shall not exceed 
150° C (302° F) under normal operating 
conditions. 



5. Electrical Clearances 

Minimum clearances 
posed electrical conductor 
explosion-proof enclosures 
listed in Table A-1. 



between 

surfaces 
shall be 



ex- 
in 



TABLE A-1, 



Minimum clearances 



COMMENT: No change is recommended 
to the current requirements of 30 
CFR 18.23. Load centers must be 
designed so as to adequately dissi- 
pate the heat generated by the 
transformer without exceeding the 
surface temperature requirements. 



Voltage range, V 

8,000 to 15,000 , 

5,000 to 8,000 , 

2,000 to 5,000 , 

1,000 to 2,000 , 



Clearance, in 

7 
4 
3 
2 



38 



6. Insulating Materials Used in 
Explosion-Proof Enclosures 

a. Inorganic insulating materials 
shall be used, where feasible, in prefer- 
ence to organic plastic insulating 
materials. 

b. Insulators conqjosed of organic 
plastic materials shall not be used as 
bushings or supports for bus bars or in 
other locations where potentially danger- 
ous short circuits might occur. 

c. Where the use of organic plastic 
insulating materials cannot be avoided, 
the following conditions shall be met: 

(1) The volume of such materials 
used shall be kept to a minimum. 



7. Gaskets and Sealed Enclosures 

a. Gaskets shall be used in accord- 
ance with 30 CFR 18.27. 

b. Hermetically sealed (welded) en- 
closures, pressurized with inert gas or 
other special atmosphere, may be used for 
transformers if 

(1) Means are provided for sens- 
ing the loss of an effective seal 
and automatically disconnecting the 
power supply from the enclosure if 
the seal is lost. The methods se- 
lected for this purpose shall pre- 
vent reapplication of power to the 
transformer until the proper atmos- 
phere and an effective seal are 
restored. 



(2) Those materials used shall 
be highly resistant to electrical 
tracking and arcing. 

(3) A detection device shall be 
provided that will operate to remove 
the power incoming to the enclosure 
before decomposition of the insulat- 
ing material due to • an electrical 
fault leads to hazardous conditions. 

COMMENT: A material will be deemed 
highly resistant to electrical 
tracking if it has a con5)arative 
tracking index (CTI) of not less 
than 250. Materials will be 
deemed highly resistant to elec- 
trical arcing if they pass the 
fuse wire arc test — described by 
J. N. Hardwich in "An Improved Fuse 
Wire Arc Test Including a Pro- 
posed Specification," The Elec- 
trical Research Association, Report 
No. 5078, 1964~or an equally 
effective test recognized by MSHA. 

The detection device specified in 
paragraph c(3) may operate on pres- 
sure rise, temperature rise, detec- 
tion of the products of insulator 
decomposition, or other effective 
means. In either case, the ground 
monitor circuit can be used to re- 
move the power incoming to the 
enclosure. 



(2) Means are provided for pre- 
venting or relieving overpressuriza- 
tion of the enclosure caused by 
accidental overfilling with gas, 
operation at high temperatures, or 
internal electrical fault. 

(3) The enclosure is of substan- 
tial design and construction so as to 
prevent damage that may lead to a 
loss of seal. The minimum thickness 
of material for the walls shall be 
1/4 in. 

c. Enclosures designed in accord- 
ance with paragraph 7(b) need not be 
designed to withstand the minimum inter- 
nal pressure of 150 psig as specified in 
30 CFR 18.31(a)(1). 

d. Enclosures designed in accord- 
ance with paragraph 7(b) need not be 
explosion tested as specified in 30 
CFR 18.62 and paragraph 19 of these 
criteria. 

8. Explosion-Proof Enclosures 

a. The requirements of 30 CFR 18.31, 
18.32, and 18.33 must be met by all 
explosion-proof load center and auxiliary 
con5)onent enclosures. All welds shall be 
made in accordance with American Welding 
Society Standard AWS D14.4-77. 



39 



b. MSHA may Impose additional re- 
quirements for the use of high-voltage 
conq)onents in potted enclosures. 

COMMENTS: The problems associated 
with placing high-voltage compo- 
nents in potted enclosures should 
be explored, and adequate testing 
and approval criteria developed, 
before this type of equipment is 
accepted. 

9. Access Openings and Covers 

Access openings in esqjlos ion-proof 
load center enclosures will be permitted 
where necessary for proper maintenance 
such as tap changing and circuit breaker 
adjustment. The provisions of 30 CFR 
18.29 must be met. 



f. ICEA standards for derating am- 
pacities for cables wound on reels, and 
ICEA recommended minimum bending diam- 
eters, shall be observed. 

g. No temporary splices shall be 
used. All permanent splices and ter- 
minations shall be made in accordance 
with manufacturer's specifications by 
qualified personnel familiar with the 
techniques required for proper high- 
voltage cable installation, operation, 
and maintenance. 

11. Lead Entrances; Cable Connectors 
and Plugs 

a. The provisions of 30 CFR 18.42 — 
Explosion-proof distribution boxes — shall 
apply to explosion-proof load centers. 



10. 



High-Voltage Power Cables 



High-voltage power cables used as 
portable cables, or located where the use 
of permissible equipment is required, 
shall conform to the following: 



b. High-voltage cable connectors 
and plugs used in areas where permis- 
sible equipment is required shall meet 
the requirements of 30 CFR 18.41 and 
the test requirements specified in 
table A-3. 



a. Have each conductor of a 
current-carrying capacity consistent with 
the Insulated Cable Engineers Association 
(ICEA) standards (see table A-2). 

b. Have current-carrying conductors 
not smaller than No. 6 (AWG). 

c. Have flame-resistant properties 
(see 30 CFR 18.64). 

d. Have short-circuit protection at 
the outby (circuit-connecting) end of 
underground conductors. The fuse rating 
or breaker trip setting shall be included 
in the assembler's specifications. 

e. Have nominal outside dimensions 
and tolerances consistent with ICEA 
standards. 



c. Tests specified in paragraph (b) 
shall be performed on each high-voltage 
connector or plug intended for use on 
permissible equipment or on approved 
cables in areas where permissible equip- 
ment is required. Equipment shall be 
designed so that plugs and recepta- 
cles can be completely assembled and 
tested before mounting on the permissi- 
ble enclosure. Cable connectors that 
have been tested for use in permissible 
areas shall be clearly marked and 
identified. 

COMMENTS: The tests listed in ta- 
ble A-3 are based on work performed 
under Bureau of Mines contract 
H0377043. 



40 



TABLE A-2. - Ampacities for portable power cables, amperes per conductor 



Power conductor 
size 



Single conductor 



2,001 to 
8,000 V,l 
shielded 



8,001 to 

15,000 V,l 

shielded 



Three-conductor, 
round and flat, 
to 5,000 V, 
nonshielded 



Three conductor, round 



to 
8,000 V, 
shielded 



8,001 to 
15,000 V, 
shielded 



AWG, copper: 

8 

6 

4 

3 

2 

1 



1/0, 
2/0. 
3/0. 
4/0. 



MCM, copper: 

250 

300 

350 

400 

450 

500 

550 

600 

650 

700 

750 

800 

900 

1,000 



NAp 
112 
148 
171 
195 
225 

260 
299 
345 
400 



444 
496 
549 
596 
640 
688 
732 
779 
817 
845 
889 
925 
998 
1,061 



NAp 
NAp 
NAp 
NAp 
195 
225 

259 
298 
343 
397 



440 
491 
543 
590 
633 
678 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 



59 
79 
104 
120 
138 
161 

186 
215 
249 
287 



3^0 
357 
394 
430 
460 
487 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 



NAp 
93 
122 
140 
159 
184 

211 
243 
279 
321 



355 
398 

435 
470 
503 
536 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 



NAp 
NAp 
NAp 
NAp 
164 
187 

215 
246 
283 
325 



359 

NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
NAp 
N^ 



NAp Not applicable. 

^Based on single isolated cable in air operated with open-circuited shield. 



NOTE. 



-These ampacities are based on a conductor tenq)erature of 90° C and an ambient 
temperature of 40° C. 



TABLE A-3. - Standard dielectric tests for high-voltage cable plugs 
and connectors used in areas where permissible 
equipment is required 



Test 


Test voltage, kV 




8.7-kV class 


15-kV class 




27 
15 
75 

7 
40 


35 


6-hr withstand value do 

Impulse withstand peak value do 

Corona (partial-discharge) extinction 


25 
95 

11 


15-min dc withstand value do 


50 



41 



the tech- 
1.38 may be 
between the 

to prevent 



12. Leads Through Common Walls Between 
Explosion-Proof Enclosures 

Insulated bushings with proper volt- 
age rating and current-carrying capaci- 
ties may be used in the common wall 
between two explosion-proof enclosures. 
When insulated wires or cables are ex- 
tended through a common wall between two 
explosion-proof enclosures, 
niques described in 30 CFR II 
en5)loyed provided the seal 
two enclosures is sufficient 
propagation of an explosion from one en- 
closure to the other. Wires and cables 
shall be mechanically secured in open 
areas of the enclosure and in passageways 
between enclosures to prevent excessive 
movement in the event of high current 
flows. 

13. Openings Through Common Walls 
Between Explosion-Proof 
Enclosures 

As provided in 30 CFR 18.38(e), un- 
sealed openings through common walls 
between explosion-proof enclosures shall 
be large enough to prevent pressure 
piling. Partitions subdividing single 
enclosures shall not be used. Internal 
coiq)onents shall be arranged so as not 
to effectively divide the interior of 
the enclosure into pockets joined by re- 
stricted passages, 

COMMENTS: Pressure piling is a 
conplex phenomenon that can oc- 
cur when gas in a portion of an 
explosion-proof enclosure is com- 
pressed before being ignited. The 
resulting pressure rise may be much 
greater than would normally be ex- 
pected from a methane-air ignition. 
Subdividing enclosures into com- 
partments connected by narrow pas- 
sages, either intentionally or by 
careless component placements, can 
result in pressure piling and 
should be avoided. 



It is not always possible to 
predict exactly when, or to what 
degree, pressure piling may occur. 
However, its presence can be 
detected in the explosion tests 
specified in 30 CFR 18.62. When a 
pressure exceeding 125 psig is 
developed during explosion tests 
(which would indicate that pressure 
piling has occurred) , MSHA can 
reject the enclosure unless con- 
structional changes are made that 
result in a reduction of pressure 
to 125 psig or less, or the enclo- 
sure withstands a dynamic pressure 
of twice the highest value recorded 
in the initial test. 

14. Voltage Limitation 

Load centers with nameplate ratings 
in excess of 4,160 V, but less than 
15,000 V, may be approved as permissible 
if the applicable requirements of 30 CFR 
18, and the additional requirements con- 
tained in these criteria, are met. 

COMMENT: The following changes to 
30 CFR 18.47 are recommended: 

• In 30 CFR 18.47(d), delete the 
words "but not exceeding 4,160 
V." 

• 30 CFR 18.47(d)(3) should be 
changed to read: "All high 
voltage switchgear and con- 
trols for equipment having a 
nameplate rating exceeding 
1,000 V are approved (permis- 
sible) for use in gassy mines 
or tunnels, or are certified 
as suitable for incorporation 
in a machine to be submitted 
for approval, or are located 
remotely and operated by 
remote control at the main 
equipment. Potential for re- 
mote control shall not exceed 
120 V." 



42 



• 30 CFR 18.47(d)(5) should be 
changed to read: "Portable 
(trailing) cable for equip- 
ment with nameplate ratings 
greater than 1,000 V shall 
include grounding conductors, 
a ground-check conductor, and 
grounded metallic shields 
around each power conductor 
and shall be adequately con- 
structed and insulated for the 
applied voltage," 

30 CFR 18.47(d)(6) reserves the 
right for MSHA to require "addi- 
tional safeguard" for high-voltage 
equipment. These criteria repre- 
sent those additional safeguards 
for high-voltage load centers (up 
to 15 kV and 2,000 kVA). Similar 
criteria should be developed, as 
needed, for other types of high- 
voltage, permissible equipment 
(for example, motors and motor 
starters) . 

15. Electrical Protective Devices 

a. High-voltage circuits connected 
to permissible load centers shall be 
protected by modern, high-speed circuit 
breakers equipped with devices to pro- 
vide protection against undervoltage, 
grounded phase, short circuit, and 
overcurrent. 

b. Upon detection of a ground fault 
in a circuit supplying power to high- 
voltage, permissible equipment, the cir- 
cuit shall be deenergized and remain so 
until the ground fault is cleared. In no 
instance shall such a circuit be ener- 
gized while a phase conductor remains 
grounded. 

c. All equipment intended to break 
current at fault levels shall have an in- 
terrupting rating sufficient for the sys- 
tem voltage and the current that is 
available at the line terminals of the 
equipment. Equipment intended to break 
current at other than fault levels shall 
have an interrupting rating at system 
voltage sufficient for the current that 
must be intermapted. 



d. The overcurrent protective de- 
vices, the total impedance, the con5)onent 
short-circuit withstand ratings, and 
other characteristics of the circuit to 
be protected shall be so selected and 
coordinated as to permit the circuit pro- 
tective devices to clear a fault without 
the occurrence of extensive damage to the 
electrical components of the circuit. 

e. Additional coordination shall be 
provided between the electrical circuit 
characteristics and protective devices 
and the design parameters (for example, 
strength, free volume) of the explosion- 
proof enclosure to prevent damage to the 
enclosure should an electrical fault oc- 
cur. Precautionary measures may include 



(1) Limitation 
fault energy. 



of 



available 



(2) Provision of adequate free 
volume within the enclosure, 

(3) Inclusion of devices which 
detect hazardous pressure and tem- 
perature rises, ozone, or visible 
arc radiation, and/or 

(4) Use of devices or tech- 
niques designed to vent or limit in- 
ternal pressure. 

COMMENT: High-voltage ac circuit 
breakers are available with inter- 
rupting speeds of 1, 2, 3, 5, and 8 
cycles. A description of various 
types and a history of the develop- 
ment of such breakers can be found 
in the "Standard Handbook for Elec- 
trical Engineers," Fink and Beaty, 
11th ed. In addition, vacuum cir- 
cuit breakers with interr^ipting 
speeds of less than 1 cycle are 
available in the voltage range of 
interest to this program. These 
breakers are in common use through- 
out the mining industry today and 
offer other advantages (for exam- 
ple, compact size, low maintenance, 
enclosed contacts) which are desir- 
able for mine electrical equipment 
in general, and permissible equip- 
ment in particular. 



I 



43 



Paragraph 15(b) is included in 
recognition of the fact that some 
mine electrical systems have, in 
the past, been allowed to operate 
for long periods (hours) with one 
phase of the power system grounded. 
This is usually done to avoid shut- 
down of an entire mine while a sin- 
gle ground fault is being located 
and repaired. However, continued 
operation with a grounded phase 
increases the electrical stress on 
the system insulation and, with 
the occurrence of a second fault, 
can give rise to a double phase- 
to-ground fault situation. This 
defeats the purpose of ground 
shields, barriers, and other forms 
of phase segregation provided in 
shielded cables and permissible 
high-voltage enclosures. If per- 
mission is ever granted to energize 
and operate a power system before 
clearing a ground fault, it should 
be conditioned on the removal from 
the circuit of equipment of the 
type covered by these approval 
criteria. 

Devices other than high-speed 
breakers are available to limit 
fault energy. Most notable of 
these is the current-limiting fuse. 
This type of protection is used 
extensively in the utility industry 
to prevent overpressurization of 
transformer enclosures due to 
internal faults. However, the 
current-limiting characteristics 
of these fuses does not come into 
play unless several (20 to 30) mul- 
tiples of rated current is availa- 
ble from the circuit in the event 
of a fault. In most mine high- 
voltage power systems, the overall 



impedance of the circuit will limit 
fault current availability to a few 
thousand amps. Only in locations 
where abnormally high fault cur- 
rents are available (for example, 
very close to the main mine sub- 
station) would current-limiting 
fuses actually provide the kind 
of protection for which they are 
designed. 

Data from testing to be performed 
later in this program may shed 
additional light on proper coordi- 
nation of circuit electrical char- 
acteristics and enclosure design 
specifications. 

, High-Voltage Circuit Design 



a. High-voltage circuits and compo- 
nents in permissible enclosures shall 
conform with accepted practices and stan- 
dards of high-voltage design for the 
appropriate voltage class. 

b. Ground barrier and shields shall 
be used when possible to minimize the 
occurrence of phase-to-phase faults with- 
in enclosures. 

17. Component Placement 

a. High-voltage electrical compo- 
nents located in explosion-proof enclo- 
sures shall not be placed in the same 
plane as the flange gap. 

b. Components located in tight, 
explosion-proof enclosures shall not be 
placed in such a way as to effectively 
subdivide or create "conq)artments" or 
"pockets" within the enclosure which 
might give rise to pressure piling upon 
ignition of a methane-air mixture. 



PART C~INSPECTIONS AND TESTS 



18. Inspections 

Inspections specified in 30 
18.60 and 18.61 shall be required 



CFR 

for 

Additional 



requirements of parts A and B of these 
criteria shall also be performed. These 
include 

a. Examination of items listed on 



permissible load centers. 

inspections which take into account the the factory inspection form. 



44 



b. Examination for the use 
proper insulating materials. 



of 



c. Examination for adequacy and 
proper installation and operation of all 
electrical and mechanical protective 
devices. 

d. Examination for areas of possi- 
ble excessive electrical stress. 



(4) Fault current — fault cur- 
rent shall be increased in succes- 
sive increments of 1,000 A until an 
excessive pressure rise (>125 psig) 
is measured in the enclosure, or to 
a maximum of 10,000 A. The enclo- 
sure shall be approved for use in 
circuits with available fault cur- 
rents less than the maximum fault 
current used in this test. 



COMMENT: In examining for areas 
of possible excessive electrical 
stress, MSHA may want to use one 
of a number of standard corona 
(partial discharge) detection and 
measurement techniques (for exam- 
ple, IEEE Standard 454—1973 or 
ASTM Standard D 1868-73). Such 
testing might also be required by 
the manufacturer as part of the 
quality assurance provisions (see 
paragraph 3 of these criteria). 

19. Tests To Determine Explosion-Proof 
Characteristics 

a. With exception of the hermet- 
ically sealed enclosures referred to in 
paragraph 7 (b, c, and d) of these cri- 
teria, all permissible enclosures used 
with load centers or related equipment 
must pass the explosion tests specified 
in 30 CFR 18.62. 



c. All or any part of the addi- 
tional tests specified in paragraph 19 
(b) may be waived by MSHA for equipment 
meeting all other requirements of these 
criteria and of 30 CFR 18, if such equip- 
ment is provided with one or more of the 
following design features: 

(1) An enclosure containing 
more than one phase of a high- 
voltage circuit is designed and 
constructed so as to preclude the 
possible occurrence of a phase-to- 
phase arcing fault (for example, 
complete phase segregation or 
shielding is provided). 

(2) An enclosure is equipped 
with approved vents or pressure re- 
lief devices such that no pressures 
greater than 15 psig are measured in 
the methane-air explosion tests re- 
quired by 30 CFR 18.62. 



b. Enclosures containing more than 
one phase of a high-voltage circuit must 
meet the requirements of 30 CFR 18.62 
when the methane-air mixture in the en- 
closure is ignited by a high-voltage, 
phase-to-phase arcing fault. The fault 
used in this test shall have the follow- 
ing specifications: 

(1) Fault duration — 15 cycles 
(0.250 sec). 



(2) Source voltage — rated 
tem voltage. 



sys- 



(3) Fault arc length — fault arc 
length (electrode spacing) shall be 
equal to the minimum clearances 
specified in paragraph 5 of these 
criteria for the appropriate system 
voltage. 



(3) An enclosure containing 
more than one phase of a high- 
voltage circuit is provided with 
a minimum free internal volume of 
1 m3. 

COMMENT: The design of high- 
voltage, metal-enclosed switchgear 
so that each phase is enclosed 
in a separate metal housing, with 
an air space provided between 
the housings, is considered to be 
the safest, most practical, and 
most economical way of prevent- 
ing phase-to-phase short-circuit 
faults through construction meth- 
ods. Therefore, the additional 
testing described in paragraph 
19(b) is required only for enclo- 
sures containing more than one 
phase. 



45 



Other design techniques may also 
provide a high degree of safety; 
they are listed in paragraphs 19(c) 
1, 2, and 3. Paragraph 19(c) gives 
MSHA the prerogative to waive arc 
tests for such designs. It should 
be noted, however, that MSHA re- 
tains the right to verify, by test- 
ing, the degree of protection 
afforded by these design features, 
and is free to exercise that right 
until sufficient experience with 
such testing is gained. 



subject to revision, pending re- 
ceipt of data from testing to be 
performed later in this program. 

The use of pressure relief vents 
[paragraph 19(c)(2)] makes field 
inspection more difficult than the 
simple feeler-gage test now used 
for explosion-proof housings. It 
may be desirable to develop a new 
test to insure that the vents have 
not become clogged once the unit is 
in operation. 



The minimum free volume specified 
in paragraph 19(c)(3) — 1 m^ — is 



46 



APPENDIX B.-CFR 30, SUBCHAPTER D-ELECTRICAL EQUIPMENT, LAMPS, METHANE 
DETECTORS; TESTS FOR PERMISSIBILITY, FEES; PARTS 18 AND 75 



PART 18— ELECTRIC MOTOR-DRIVEN 
MINE EQUIPMENT AND ACCESSO- 
RIES 

Subpart A — General Provisions 

Sec. 

18.1 Purpose. 

18.2 Definitions. 

18.3 Consultation. 

18.4 Equipment for which approval will be 
issued. 

18.5 Equipment for which certification will 
be issued. 

18.6 Applications. 

18.7 Fees. 

18.8 Date for conducting investigation and 
tests. 

18.9 Conduct of investigations and tests. 

18.10 Notice of approval or disapproval. 

18.11 Approval plate. 

18.12 Letter of certification. 

18.13 Certification plate. 

18.14 Identification of tested noncertified 
explosion-proof enclosures. 

18.15 Changes after approval or certifica- 
tion. 

18.16 Withdrawal of approval, certifica- 
tion, or acceptance. 

Subport B — Construction and Design 
Requirements 



20 Quality of material, workmanship, 
and design. 

21 Machines equipped with powered 
dust collectors. 

22 Boring-type machines equipped for 
auxiliary face ventilation. 

23 Limitation of external surface tem- 
peratures. 

24 Electrical clearances. 

25 Combustible gases from insulating 
material. 

26 Static electricity. 

27 Gaskets. 

28 Devices for pressure relief, ventila- 
tion, or drainage. 

29 Access openings and covers, including 
unused lead-entrance holes. 

30 Windows and lenses. 

31 Enclosures— joints and fastenings. 

32 Fastenings— additional requirements. 

33 Finish of surface joints. 

34 Motors, 

35 Portable (trailing) cables and cords. 

36 Cables between machine compo- 
nents. 

37 Lead entrances. 

38 Leads through common walls. 

39 Hose conduit. 

40 Cable clamps and grips. 

41 Plug and receptacle-type connectors. 

42 Explosion-proof distribution boxes. 

43 Explosion-proof splice boxes. 

44 Battery boxes and batteries (exceed- 
ing 12 volts). 

45 Cable reels. 

46 Headlights. 

47 Voltage limitation. 

48 Circuit-interrupting devices. 

49 Connection boxes on machines. 

50 Protection against external arcs and 
sparks. 

51 Electrical protection of circuits and 
equipment. 

52 Renewal of fuses. 



Subpart C — Inspections and Tests 

Sec. 

18.60 Detailed inspection of components. 

18.61 Final inspection of complete ma- 
chine. 

18.62 Tests to determine explosion-proof 
characteristics. 

18.63 Tests of battery boxes. 

18.64 Tests for flame resistance of cables. 

18.65 Flame test of conveyor belting and 
hose. 

18.66 Tests of windows and lenses. 

18.67 Static-pressure tests. 

18.68 Tests for intrinsic safety. 

18.69 Adequacy tests. 

Subpart D — Machines Assembled With Certi- 
fied or Explosion-Proof Components, Field 
Modifications of Approved Machines, and 
Permits To Use Experimental Equipment 

18.80 Approval of machines assembled 
with certified or explosion-proof compo- 
nents. 

18.81 Field modification of approved (per- 
missible) equipment: application for ap- 
proval of modification; approval of plans 
for modification before modification. 

18.82 Permit to use experimental electric 
face equipment in a gassy mine or 
tunnel. 

Appendix I 
Appendix II 

Subpart E — Field Approval of Electrically 
Operated Mining Equipment 

18.90 Purpose. 

18.91 Electric equipment for which field 
approvals will be issued. 

18.92 Quality of material and design. 

18.93 Application for field approval; filing 
procedures. 

18.94 Application for field approval; con- 
tents of application. 

18.95 Approval of machines constructed of 
components approved, accepted or certi- 
fied under Bureau of Mines Schedule 
2D, 2E, 2F, or 2G. 

18.96 Preparation of machines for inspec- 
tion; requirements. 

18.97 Inspection of machines; minimum re- 
quirements. 

18.98 Enclosures, joints, and fastenings; 
pressure testing. 

18.99 Notice of approval or disapproval; 
letters of approval and approval plates. 



Subpart A — General Provisions 

§ 18.1 Purpose. 

The regulations in this part set forth 
the requirements to obtain MSHA: (a) 
Approval of electrically operated ma- 
chines and accessories intended for 
use in gassy mines or tunnels, (b) certi- 
fication of components intended for 
use on or with approved machines, (c) 
permission to modify the design of an 
approved machine or certified compo- 
nent, (d) acceptance of flame-resistant 



cables, hoses, and conveyor belts, (e) 
sanction for use of experimental ma- 
chines and accessories in gassy mines 
or tunnels; also, procedures for apply- 
ing for such approval, certification, ac- 
ceptance for listing; and fees. 

§ 18.2 Definitions. 

• • • 

"Approval" means a formal docu- 
ment issued by MSHA which states 
that a completely assembled electrical 
machine or accessory has met the ap- 
plicable requirements of this part and 
which authorizes the attachment of 
an approval plate so indicating. 

"Approval plate" means a metal 



"Certification" means a formal writ- 
ten notification, issued by MSHA, 
which states that an electrical compo- 
nent complies with the applicable re- 
quirements of this part and, therefore, 
is suitable for incorporation in ap- 
proved (permissible) equipment. 



"Component" means an integral 
part of an electrical machine or acces- 
sory that is essential to the function- 
ing of the machine or accessory. 



"Distribution box" means an enclo- 
sure through which one or more port- 
able cables may be connected to a 
source of electrical energy, and which 
contains a short-circuit protective 
device for each outgoing cable. 
• • • 

"Explosion-proof enclosure" means 
an enclosure that complies with the 
applicable design requirements in Sub- 
part B of this part and is so construct- 
ed that it will withstand internal ex- 
plosions of methane-air mixtures: (1) 
Without damage to or excessive distor- 
tion of its walls or cover(s), and (2) 
without ignition of surrounding meth- 
ane-air mixtures or discharge of flame 
from inside to outside the enclosure. 



"Flame-arresting path" means two 
or more adjoining or adjacent surfaces 
between , which the escape of flame is 
prevented. 



"Gassy mine" means a coal mine 
classed as "gassy" by MESA or by the 
State in which the mine is situated. 

"Incendive arc or spark" means an 
arc or spark releasing enough electri- 
cal or thermal energy to ignite a flam- 
mable mixture of the most easily ignit- 
able composition. 



47 



"Intrinsically safe" means incapable 
of releasing enough electrical or ther- 
mal energy under normal or abnormal 
conditions to cause ignition of a flam- 
mable mixture of methane or natural 
gas and air of the most easily ignitable 
composition. 

• • • 
"Permissible equipment" means a 
completely assembled electrical ma- 
chine or accessory for which a formal 
approval has been issued, as author- 
ized by the Administrator, Mining En- 
forcement and Safety Administration 
under the Federal Coal Mine Health 
and Safety Act of 1969 (Pub. L. 91-173, 
30 U.S.C. 801 or, after March 9. 1978, 
by the Assistant Secretary under the 
Federal Mine Safety and Health Act 
of 1977 (Pub. L. 91-173, as amended by 
Pub. L. 95-164, 30 U.S.C. 801). 



"Portable cable", or "trailing cable" 
means a flame-resistant, flexible cable 
or cord through which electrical 
energy is transmitted to a permissible 
machine or accessory. (A portable 
cable is that portion of the power- 
supply system between the last short- 
circuit protective device, acceptable to 
MSHA, in the system and the machine 
or accessory to which it transmits elec- 
trical energy.) 

"Portable equipment" means equip- 
ment that may be moved frequently 
and is constructed or mounted to fa- 
cilitate such movement. 

"Potted component" means a compo- 
nent that is entirely embedded in a so- 
lidified insulating material within an 
enclosure. 

"Pressure piling" means the develop- 
ment of abnormal pressure as a result 
of accelerated rate of burning of a gas- 
air mixture. (Frequently caused by re- 
stricted configurations within enclo- 
sures.) 



§ 18.6 Applications. 



• • • 

(e) Drawings, drawing lists, specifica- 
tions, wiring diagram, and descriptions 
shall be adequate in number and 
detail to identify fully the complete 
assembly, component parts, and subas- 
semblies. Drawings shall be titled, 
numbered, dated and shall show the 
latest revision. Each drawing shall in- 
clude a warning statement that 
changes in design must be authorized 
by MSHA before they are appliea to 
approved equipment. When intrinsi- 
cally safe circuits are incorporated in a 
machine or accessory, the wiring dia- 
gram shall include a warning state- 



ment that any change(s) in the intrin- 
sically safe circuitry or components 
may result in an unsafe condition. The 
specifications shall include an assem- 
bly drawing(s) (see Figure 1 in Appen- 
dix II) showing the overall dimensions 
of the machine and the identity of 
each component part which may be 
listed thereon or separately, as in a 
bill of material (see Figure 2 in Appen- 
dix II). MSHA may accept photo- 
graphs (minimum size 8" x 10 'A" ) in 
lieu of assembly drawing(s). Purchased 
parts shall be identified by the manu- 
facturer's name, catalog number(s), 
and rating(s). In the case of standard 
hardware and miscellaneous parts, 
such as insulating pieces, size and kind 
of material shall be specified. All 
drawings of component parts submit- 
ted to MSHA shall be identical to 
those used in the manufacture of the 
parts. Dimensions of parts designed to 
prevent the passage of flame shall 
specify allowable tolerances. A nota- 
tion "Do Not Drill Through" or equiv- 
alent should appear on drawings with 
the specifications for all "blind" holes, 
(f) MSHA reserves the right to re- 
quire the applicant to furnish supple- 
mentary drawings showing sections 
through complex flame-arresting 
paths, such as labyrinths used in con- 
junction with ball or roller bearings, 
and also drawings containing dimen- 
sions not indicated on other drawings 
submitted to MSHA. 



(j) The applicant shall submit a 
sample caution statement (see Figure 
3 in Appendix II) specifying the condi- 
tions for maintaining permissibility of 
the equipment. 

(k) The applicant shall submit a fac- 
tory-inspection form (see Figure 4 in 
Appendix II) used to maintain quality 
control at the place of manufacture or 
assembly to insure that component 
parts are made and assembled in strict 
accordance with the drawings and 
specifications covering a design sub- 
mitted to MSHA for approval or certi- 
fication. 



§ 18.11 Approval plate. 



(c) The approval plate identifies as 
permissible the machine or accessory 
to which it is attached, and use of the 
approval plate obligates the applicant 
to whom the approval was issued to 
maintain in his plant the quality of 
each complete assembly and guaran- 
tees that the equipment is manufac- 
tured and assembled according to the 
drawings, specifications, and descrip- 
tions upon which the approval and 
subsequent extension(s) of approval 
were based. 



S 18.12 Letter of certification. 



(b) A letter of certification will be 
accompanied by a list of drawings, 
specifications, and related material 
covering the details of design and con- 
struction of a component upon which 
the letter of certification is based. Ap- 
plicants shall keep exact duplicates of 
the drawings, specifications, and de- 
scriptions that relate to the compo- 
nent for which a letter of certification 
has been issued; and the drawings and 
specifications shall be adhered to ex- 
actly in production of the certified 
component. 



Subpart B — Construction and Design 
Requirements 

S 18.20 Quality of material, workmanship, 
and design. 

(a) Electrically operated equipment 
intended for use in coal mines shall be 
rugged in construction and shall be de- 
signed to facilitate inspection and 
maintenance. 

(b) MSHA will test only electrical 
equipment that in the opinion of its 
qualified representatives is construct- 
ed of suitable materials, is of good 
quality workmanship, based on sound 
engineering principles, and is safe for 
its intended use. Since all possible de- 
signs, circuits, arrangements, or combi- 
nations of components and materials 
cannot be foreseen, MSHA reserves 
the right to modify design, construc- 
tion, and test requirements to obtain 
the same degree of protection as pro- 
vided by the tests described in Subpart 
C of this part. 



(d) Flange joints and lead entrances 
shall be accessible for field inspection, 
where practicable. 

• • • 

S 18.2,'J Limitation of external surface 
temperatures. 

The temperature of the external 
surfaces of mechanical or electrical 
components shall not exceed 150° C. 
(302° F.) under normal operating con- 
ditions. 

§ 18.24 Electrical clearances. 

The clearance between live parts and 
casings shall be sufficient to minimize 
the possibility of arcs striking the cas- 
ings. Where space is limited, the 
casing shall be lined with adequate in- 
sulation. 

8 18.25 Combustible gases from insulating 
material. 

(a) Insulating materials that give off 
flammable or explosive gases when de- 



48 



composed electrically shall not be used 
within enclosures where the materials 
are subjected to destructive electrical 
action. 

(b) Parts coated or impregnated with 
insulating materials shall be heat- 
treated to remove any combustible 
solvent(s) before assembly in an explo- 
sion-proof enclosure. Air-drying insu- 
lating materials are excepted. 

• • • 
§ 18.27 Gaskets. 

A gasket(s) shall not be used be- 
tween any two surfaces forming a 
flame-arresting path except as follows: 

(a) A gasket of lead, elastomer, or 
equivalent will be acceptable provided 
the gasket does not interfere with an 
acceptable metal-to-metal joint. 

(b) A lead gasket(s) or equivalent 
will be acceptable between glass and a 
hard metal to form all or a part of a 
flame-arresting path. 

§ 18.28 Devices for pressure relief, ventila- 
tion, or drainage. 

(a) Devices for installation on explo- 
sion-proof enclosures to relieve pres- 
sure, ventilate, or drain will be accept- 
able provided the length of the flame- 
arresting path and the clearances or 
size of holes in perforated metal will 
prevent discharge of flame in explo- 
sion tests. 

(b) Devices for pressure relief, venti- 
lation, or drainage shall be construct- 
ed of materials that resist corrosion 
and distortion, and be so designed that 
they can be cleaned readily. Provision 
shall be made for secure attachment 
of such devices. 

(c) Devices for pressure relief, venti- 
lation, or drainage will be acceptable 
for application only on enclosures 
with which they are explosion tested. 

§ 18.29 Access openings and covers, in- 
cluding unused lead-entrance holes. 

(a) Access openings in explosion- 
proof enclosures will be permitted 
only where necessary for maintenance 
of internal parts such as motor 
brushes and fuses. 

(b) Covers for access openings shall 
meet the same requirements as any 
other part of an enclosure except that 
threaded covers shall be secured 
against loosening, preferably with 
screws having heads requiring a spe- 
cial tool. (See Figure 1 in Appendix 
II.) 

(c) Holes in enclosures that are pro- 
vided for lead entrances but which are 
not in use shall be closed with metal 
plugs secured by spot welding, brazing, 
or equivalent. (See Figure 10 in Ap- 
pendix II.) 

§ 18.30 Windows and lenses. 

(a) MSHA may waive testing of ma- 
terials for windows or lenses except 
headlight lenses. When tested, materi- 



al for windows or lenses shall meet the 
test requirements prescribed in § 18.66 
and shall be sealed in place or pro- 
vided with flange joints in accordance 
with § 18.31. 

(b) Windows or lenses shall be pro- 
tected from mechanical damage by 
structural design, location, or guard- 
ing. Windows or lenses, other than 
headlight lenses, having an exposed 
area greater than 8 square inches, 
shall be provided with guarding or 
equivalent. 

§ 18.31 Enclosures— joints and fastenings. 
(a) Explosion-proof enclosures: 

(1) Cast or welded enclosures shall 
be designed to withstand a minimum 
internal pressure of 150 pounds per 
square inch (gage). Castings shall be 
free from blowholes. 

(2) Welded joints forming an enclo- 
sure shall have continuous gas-tight 
welds. All welds shall be made in ac- 



cordance with American Welding Soci- 
ety standards. 

(3) External rotating parts shall not 
be constructed of aluminum alloys 
containing more than 0.5 percent mag- 
nesium. 

(4) MSHA reserves the right to re- 
quire the applicant to conduct static- 
pressure tests on each enclosure when 
MSHA determines that the particular 
design will not permit complete visual 
inspection or when the joint(s) form- 
ing an enclosure is welded on one side 
only (see § 18.67). 

(5) Threaded covers shall be de- 
signed with Class 1 (coarse, loose fit- 
ting) threads. The flame-arresting 
path of threaded joints shall conform 
to the requirements of paragraph (a) 
(6) of this section. 

(6) Enclosures shall meet the follow- 
ing requirements based on the internal 
volumes of the empty enclosure. 



Less than 45 cu 



Volume of empty enclosure 
45 to 124 cu. in.. 



+- 



Minimum thickness ol matenal lor walls 

Minimum thicKness ol material lor flanges 

Minimum thickness of material for cover 

Minimum width ol |Oint— all in one plane 

Maximum clearance— loinl all in one plane 

Minimum width ol joint, portions ol which are in 

different planes— cylinders or equivalent 
Maximum clearances— joint in two or more planes. 

cylinders or equivalent 

(a) Portion perpendicular to plane 

(b) Plane portion 

Maximum bolt =• spacing— loinis all in one plane 



Maximum bolt spacing — joints, portions of which 

are in different planes. 
Minimum diameter of bolt (without regard to type of 

loint) 

Minimum thread engagement' 

Maximum diametncal clearance between bolt body 

and unthreaded holes through which it passes". 
Minimum distance Irom interior of enclosure to the 

edge of a bolt hole: 

Joint — minimum width 1" 

Joint- less than 1" wide 



More than 124 cu. 





Cylindrical Joints 








Shafts centered by ball or roller bearings: 


Vi" 


y." 


0.015". 






010" 


0125" 




Shafts through journal beanngs: "> 
















Other than shafts. 

Minimum length of flame-arresting path 






0.003" 




0015" 


002 ' 













' V3 2-inch less is allowable for machining rolled plate. 

2 ViB-inch less is allowable for machining rolled plate. 

' If only two planes are involved, neither portion of a joint shall be less than 'A-inch wide, unless the wider portion conforms 
to the same requirements as those lor a joint that is all m one plane If more than two planes are involved (as in labynnlhs or 
tongue-and-groove joints) the combined lengths of those portions having prescribed clearances will t>e considered 

'The allowable diametrical clearance is 008 inch when the portion perpendicular to the plane portion is V, inch of or 
greater in length If the perpendicular portion is more than 'A incti but less than 'A inch wide, the diametncal clearance shall 
not exceed 006 inch 

> Where the term "bolt" is used, it refers to a machine bolt or a cap screw, and for either of these studs may be substituted 
provided the studs bottom in blind holes, are completely welded in place, or the bottom of the hole is closed with a secured 
plug Bolts shall be provided at all corners 

^ Adequacy of bolt spacing will be judged on basis of size and configuration of the enclosure, strength of matenals. and 
explosion test results 

' In general, minimum thread engagement shall be equal to or greater than the diameter ol the bolt specified 

"Threaded holes for fastening bolts shall be machined to remove burrs or projections that affect planarity of a surface 
forming a flame-arresting path. 

"Less than Vn-inch ('/<-inch minimum) will be acceptable provided the diametncal clearance for fastening bolts does not 
exceed Vsz inch 

'"Shafts or operating rods through journal bearings shall be not less than '/.-inch in diameter. The length of (it shall not be 
reduced when a pushbutton is depressed Operating rods shall have a shoulder or head on the portion inside the enclosure 
Essential pans riveted or bolted to the inside portion will be acceptable in lieu of a head or shoulder, but cotter pins and 
similar devices will not be acceptable. 



A9 



(b) Enclosures for potted compo- 
nents: Enclosures shall be rugged and 
constructed with materials having 75 
percent, or greater, of the thickness 
and flange width specified in para- 
graph (a) of this section. These enclo- 
sures shall be provided with means for 
attaching hose conduit, unless energy 
carried by the cable is intrinsically 
safe. 

(c) No assembly will be approved 
that requires the opening of an explo- 
sion-proof enclosure to operate a 
switch, rheostat, or other device 
during normal operation of a machine. 

§ 18.32 Fastenings — additional require- 
ments. 

(a) Bolts, screws, or studs shall be 
used for fastening adjoining parts to 
prevent the escape of flame from an 
enclosure. Hinge pins or clamps will be 
acceptable for this purpose provided 
MSHA determines them to be equally 
effective. 

(b) Lockwashers shall be provided 
for all bolts, screws, and studs that 
secure parts of explosion-proof enclo- 
sures. Special fastenings designed to 
prevent loosening will be acceptable in 
lieu of lockwashers, provided MSHA 
determines them to be equally effec- 
tive. 

(c) Fastenings shall be as uniform in 
size as practicable to preclude improp- 
er assembly. 

(d) Holes for fastenings shall not 
penetrate to the interior of an explo- 
sion-proof enclosure, except as pro- 
vided in paragraph (a)(9) of § 18.34, 
and shall be threaded to insure that a 
specified bolt or screw will not bottom 
even if its lockwasher is omitted. 

(e) A minimum of Va inch of stock 
shall be left at the center of the 
bottom of each hole drilled for fasten- 
ings. 

(f) Fastenings used for joints on ex- 
plosion-proof enclosures shall not be 
used for attaching nonessential parts 
or for making electrical connections. 

(g) The acceptable sizes for and 
spacings of fastenings shall be deter- 
mined by the size of the enclosure, as 
indicated in § 18.31. 

(h) MSHA reserves the right to con- 
duct explosion tests with standard 
bolts, nuts, cap screws, or studs substi- 
tuted for any special high-tensile 
strength fastening(s) specified by the 
applicant. 

§ 18.33 Finish of surface joints. 

Flat surfaces between bolt holes 
that form any part of a flame-arrest- 
ing path shall be plane to within a 
maximum deviation of one-half the 
maximum clearance specified in 
§ 18.31(a)(6). All metal surfaces shall 
be finished in manufacture to not 
more than 250 microinches. A thin 
film of nonhardening preparation to 



inhibit rusting may be applied to fin- 
ished steel surfaces. 



§ 18..35 Portable (trailing) cables and 
cords. 

(a) Portable cables and cords used to 
conduct electrical energy to face 
equipment shall conform to the fol- 
lowing: 

(1) Have each conductor of a cur- 
rent-carrying capacity consistent with 
the Insulated Power Cable Engineers 
Association (IPCEA) standards. (See 
Tables 1 and 2 in Appendix I.) 

(2) Have current-carrying conductors 
not smaller than No. 14 (AWG). Cords 
with sizes 14 to 10 (AWG) conductors 
shall be constructed with heavy jack- 
ets, the diameters of which are given 
in Table 6 in Appendix I. 

(3) Have flame-resistant properties. 
(See § 18.64.) 

(4) Have short-circuit protection at 
the outby (circuit-connecting) end of 
ungrounded conductors. (See Table 8 
in Appendix I.) The fuse rating or trip 
setting shall be included in the assem- 
bler's specifications. 

(5) Ordinarily the length of a porta- 
ble (trailing) cable shall not exceed 
500 feet. Where the method of mining 
requires the length of a portable 
(trailing) cable to be more than 500 
feet, such length of cable shall be per- 
mitted only under the following pre- 
scribed conditions: 

(i) The lengths of portable (trailing) 
cables shall not exceed those specified 
in Table 9, Appendix I, titled "Specifi- 
cations for Portable Cables Longer 
Than 500 Feet." 

(ii) Short-circuit protection shall be 
provided by a protective device with 
an instantaneous trip setting as near 
as practicable to the maximum start- 
ing-current-inrush value, but the set- 
ting shall not exceed the trip value 
specified in MSHA approval for the 
equipment for which the portable 
(trailing) cable furnishes electric 
power. 

(6) Have nominal outside dimensions 
consistent with IPCEA standards. (See 
Tables 4, 5, 6, and 7 in Appendix I.) 

(7) Have conductors of No. 4 (AWG) 
minimum for direct-current mobile 
haulage units or No. 6 (AWG) mini- 
mum for alternating-current mobile 
haulage units. 

(8) Have not more than five well- 
made temporary splices in a single 
length of portable cable. 

(b) Sectionalized portable cables will 
be acceptable provided the connectors 
used inby the last open crosscut in a 
gassy mine meet the requirements of 
§ 18.41. 

(c) A portable cable having conduc- 
tors smaller than No. 6 (AWG), when 
used with a trolley tap and a rail 
clamp, shall have well insulated single 



conductors not smaller than No. 6 
(AWG) spliced to the outby end of 
each conductor. All splices shall be 
made in a workmanlike manner to 
insure good electrical conductivity, in- 
sulation, and mechanical strength. 

(d) Suitable provisions shall be made 
to facilitate disconnection of portable 
cable quickly and conveniently for re- 
placement. 

[33 FR 4660. Mar. 19, 1968; 33 PR 6343, Apr. 
26, 1968] 

§ 18.36 Cables between machine compo- 
nents. 

(a) Cables between machine compo- 
nents shall have: (1) Adequate cur- 
rent-carrying capacity for the loads in- 
volved, (2) short-circuit protection, (3) 
insulation compatible with the im- 
pressed voltage, and (4) flame-resis- 
tant properties unless totally enclosed 
within a flame-resistant hose conduit 
or other flame-resistant material. 

(b) Cables between machine compo- 
nents shall be: ( 1 ) Clamped in place to 
prevent undue movement, (2) protect- 
ed from mechanical damage by posi- 
tion, flame-resistant hose conduit, 
metal tubing, or troughs (flexible or 
threaded rigid metal conduit will not 
be acceptable), (3) isolated from hy- 
draulic lines, and (4) protected from 
abrasion by removing all sharp edges 
which they might contact. 

(c) Cables (cords) for remote-control 
circuits extending from permissible 
equipment will be exempted from the 
requirements of conduit enclosure pro- 
vided the total electrical energy car- 
ried is intrinsically safe or that the 
cables are constructed with heavy 
jackets, the sizes of which are stated 
in Table 6 of Appendix I. Cables 
(cords) provided with hose-conduit 
protection shall have a tensile 
strength not less than No. 16 (AWG) 
three-conductor, type SO cord. (Refer- 
ence: 7.7.7 IPCEA Pub. No. S-19-81, 
Fourth Edition.) Cables (cords) con- 
structed with heavy jackets shall con- 
sist of conductors not smaller than No. 
14 (AWG) regardless of the number of 
conductors. 

§ 18.37 Lead entrances. 

(a) Insulated cable(s), which must 
extend through an outside wall of an 
explosion-proof enclosure, shall pass 
through a stuffing-box lead entrance. 
All sharp edges that might damage in- 
sulation shall be removed from stuff- 
ing boxes and packing nuts. 

(b) Stuffing boxes shall be so de- 
signed, and the amount of packing 
used shall be such, that with the pack- 
ing properly compressed, the gland 
nut still has a clearance distance of Vs 
inch or more to travel without meet- 
ing interference by parts other than 
packing. (See Figures 8, 9, and 10 in 
Appendix II.) 

(c) Packing nuts and stuffing boxes 
shall be secured against loosening. 



50 



(d) Compressed packing material 
shall be in contact with the cable 
jacket for a length of not less than '/2 
inch. 

(e) Special requirements for glands 
in which asbestos-packing material is 
specified are: 

(1) Asbestos-packing material shall 
be untreated, not less than ^A 6-inch di- 
ameter if round, or not less than Vifi by 
■Vi6 inch if square. The width of. the 
space for packing material shall not 
exceed by more than 50 percent the di- 
ameter or width of the uncompressed 
packing material. 

(2) The allowable diametrical clear- 
ance between the cable and the holes 
in the stuffing box and packing nut 
shall not exceed 75 percent of the 
nom.inal diameter or width of the 
packing material. 

(f) Special requirements for glands 
in which a compressible material (ex- 
ample—synthetic elastomers) other 
than asbestos is specified, are: 

(1) The packing material shall be 
flame resistant. 

(2) The radial clearance between the 
cable jacket and the nominal inside di- 
ameter of the packing material shall 
not exceed V32 inch, based on the 
nominal specified diameter of the 
cable. 

(3) The radial clearance between the 
nominal outside diameter of the pack- 
ing material and the inside wall of the 
stuffing box (that portion into which 
the packing material fits) shall not 
exceed V32 inch. 

§ 18.38 Leads through common walls. 

(a) Insulated studs will be acceptable 
for use in a common wall between two 
explosion-proof enclosures. 

(b) When insulated wires or cables 
are extended through a common wall 
between two explosion-proof enclo- 
sures in insulating bushings, such 
bushings shall be not less than 1-inch 
long and the diametrical clearance be- 
tween the wire or cable insulation and 
the holes in the bushings shall hot 
exceed Vie inch (based on the nominal 
specified diameter of the cable). The 
insulating bushings shall be secured in 
the metal wall. 

(c) Insulated wires or cables conduct- 
ed from one explosion-proof enclosure 
to another through conduit, tubing, 
piping, or other solid-wall passageways 
will be acceptable provided one end of 
the passageway is plugged, thus isolat- 
ing one enclosure from the other. 
Glands of secured bushings with close- 
fitting holes through which the wires 
or cables are conducted will be accept- 
able for plugging. The tubing or duct 
specified for the passageway shall be 
brazed or welded into the walls of 
both explosion-proof enclosures with 
continuous gas-tight welds. 

(d) If wires and cables are taken 
through openings closed with sealing 



compounds, the design of the opening 
and characteristics of the compounds 
shall be such as to hold the sealing 
material in place without tendency of 
the material to crack or flow out of its 
place. The material also must with- 
stand explosion tests without cracking 
or loosening. 

(e) Openings through common walls 
between explosion-proof enclosures 
not provided with bushings or sealing 
compound, shall be large enough to 
prevent pressure piling. 



18.: 



Ht 



>nduit. 



Hose conduit shall be provided for 
mechanical protection of all machine 
cables that are exposed to damage. 
Hose conduit shall be flame resistant 
and have a minimum wall thickness of 
^16 inch. The flame resistance of hose 
conduit will be determined in accord- 
ance with the requirements of § 18.65. 

§ 18.40 Cable clamps and grips. 

Insulated clamps shall be provided 
for all portable (trailing) cables to pre- 
vent strain on the cable terminals of a 
machine. Also insulated clamps shall 
be provided to prevent strain on both 
ends of each cable or cord leading 
from a machine to a detached or sepa- 
rately mounted component. Cable 
grips anchored to the cable may be 
used in lieu of insulated strain clamps. 
Supporting clamps for cables used for 
wiring around machines shall be pro- 
vided in a manner acceptable to 
MSHA. 

§ 18.41 Plug and receptacle-type connec- 
tors. 

(a) Plug and receptacle-type connec- 
tors for use inby the last open crosscut 
in a gassy mine shall be so designed 
that insertion or withdrawal of a plug 
cannot cause incendive arcing or 
sparking. Also, connectors shall be so 
designed that no live terminals, except 
as hereinafter provided, are exposed 
upon withdrawal of a plug. The fol- 
lowing types will be acceptable: 

(1) Connectors in which the mating 
or separation of the male and female 
electrodes is accomplished within an 
explosion-proof enclosure. 

(2) Connectors that are mechanical- 
ly or electrically interlocked with an 
automatic circuit-interrupting device. 

(i) Mechanically interlocked connec- 
tors. If a mechanical interlock is pro- 
vided the design shall be such that the 
plug cannot be withdrawn before the 
circuit has been interrupted and the 
circuit cannot be established with the 
plug partially withdrawn. 

(ii) Electrically interlocked connec- 
tors. If an electrical interlock is pro- 
vided, the total load shall be removed 
before the plug can be withdrawn and 
the electrical energy in the interlock- 
ing pilot circuit shall be intrinsically 
safe, unless the pilot circuit is opened 
within an explosion-proof enclosure. 



(3) Single-pole connectors for indi- 
vidual conductors of a circuit used at 
terminal points shall be so designed 
that all plugs must be completely in- 
serted before the control circuit of the 
machine can be energized. 

(b) Plug and receptacle-type connec- 
tors used for sectionalizing the cables 
outby the last open crosscut in a gassy 
mine need not be explosion-proof or 
electrically interlocked provided such 
connectors are designed and construct- 
ed to prevent accidental separation. 

(c) Conductors shall be securely at- 
tached to the electrodes in a plug or 
receptacle and the connections shall 
be totally enclosed. 

(d) Molded-elastomer connectors will 
be acceptable provided: 

(1) Any free space within the plug or 
receptacle is isolated from the exterior 
of the plug. 

(2) Joints between the elastomer and 
metal parts are not less than 1 inch 
wide and the elastomer is either 
bonded to or fits tightly with metal 
parts. 

(e) The contacts of all line-side con- 
nectors shall be shielded or recessed 
adequately. 

(f) For a mobile battery-powered ma- 
chine, a plug padlocked to the recepta- 
cle will be acceptable in lieu of an in- 
terlock provided the plug is held in 
place by a threaded ring or equivalent 
mechanical fastening in addition to 
the padlock. A connector within a pad- 
locked enclosure will be acceptable. 

§ 18.42 Explosion-proof distribution boxes. 

(a) A cable passing through an out- 
side walKs) of a distribution box shall 
be conducted either through a packing 
gland or an interlocked plug and re- 
ceptacle. 

(b) Short-circuit protection shall be 
provided for each branch circuit con- 
nected to a distribution box. The cur- 
rent-carrying capacity of the specified 
connector shall be compatible with the 
automatic circuit-interrupting device. 

(c) Each branch receptacle shall be 
plainly and permanently marked to in- 
dicate its current-carrying capacity 
and each receptacle shall be such that 
it will accommodate only an appropri- 
ate plug. 

(d) Provision shall be made to relieve 
mechanical strain on all connectors to 
distribution boxes. 

S 18.43 Explosion-proof splice boxes. 

Internal connections shall be rigidly 
held and adequately insulated. Strain 
clamps shall be provided for all cables 
entering a splice box. 

• • • 

§ 18.47 Voltage limitation. 

• • • 

(d) An alternating-current machine 
shall not have a nameplate rating ex- 



51 



ceeding 660 volts, except that a ma- 
chine may have a nameplate rating 
greater than 660 volts but not exceed- 
ing 4,160 volts when the following con- 
ditions are complied with: 

(1) Adequate clearances and insula- 
tion for the particular voltage(s) are 
provided in the design and construc- 
tion of the equipment, its wiring, and 
accessories. 

(2) A continuously monitored, fail- 
safe grounding system is provided that 
will maintain the frame of the equip- 
ment and the frames of all accessory 
equipment at ground potential. Also, 
the equipment, including its controls 
and portable (trailing) cable, will be 
deenergized automatically upon the 
occurrence of an incipient ground 
fault. The ground-fault-tripping cur- 
rent shall be limited by grounding 
resistor(s) to that necessary for de- 
pendable relaying. The maximum 
ground-fault-tripping current shall not 
exceed 25 amperes. 

(3) All high voltage switch gear and 
control for equipment having a name- 
plate rating exceeding 1,000 volts are 
located remotely and operated by 
remote control at the main equipment. 
Potential for remote control shall not 
exceed 120 volts. 

(4) Portable (trailing) cable for 
equipment with nameplate ratings 
from 661 volts through 1,000 volts 
shall include grounding conductors, a 
ground check conductor, and ground- 
ed metallic shields around each power 
conductor or a grounded metallic 
shield over the assembly; except that 
on machines employing cable reels, 
cables without shields may be used if 
the insulation is rated 2,000 volts or 
more. 

(5) Portable (trailing) cable for 
equipment with nameplate ratings 
from 1,001 volts through 4,160 volts 
shall include grounding conductors, a 
ground check conductor, and ground- 
ed metallic shields around each power 
conductor. 

(6) MSHA reserves the right to re- 
quire additional safeguards for high- 
voltage equipment, or modify the re- 
quirements to recognize improved 
technology. 

§ 18.48 Circuit-interrupting devices. 

(a) Each machine shall be equipped 
with a circuit-interrupting device by 
means of which all power conductors 
can be deenergized at the machine. A 
manually operated controller will not 
be acceptable as a service switch. 

(b) When impracticable to mount 
the main-circuit-interrupting device on 
a machine, a remote enclosure will be 
acceptable. When contacts are used as 
a main-circuit-interrupting device, a 
means for opening the circuit shall be 
provided at the machine and at the 
remote contactors. 



§ 18.49 Connection boxes on machines. 

Connection boxes used to facilitate 
replacement of cables or machine com- 
ponents shall be explosion-proof. Port- 
able-cable terminals on cable reels 
need not be in explosion-proof enclo- 
sures provided that connections are 
well made, adequately insulated, pro- 
tected from damage by location, and 
securely clamped to prevent mechani- 
cal strain on the connections. 

§ 18.50 Protection against external arcs 
and sparks. 

Provision shall be made for main- 
taining the frames of all off-track ma- 
chines and the enclosures of related 
detached components at safe voltages 
by using one or a combination of the 
following: 

(a) A separate conductor(s) in the 
portable cable in addition to the power 
conductors by which the machine 
frame can be connected to an accept- 
able grounding medium, and a sepa- 
rate conductor in all cables connecting 
related components not on a common 
chassis. The cross-sectional area of the 
additional conductor(s) shall not be 
less than 50 percent of that of one 
power conductor unless a ground-fault 
tripping relay is used, in which case 
the minimum size may be No. 8 
(AWG). Cables smaller than No. 6 
(AWG) shall have an additional 
conductor(s) of the same size as one 
power conductor. 

(b) A means of actuating a circuit-in- 
terrupting device, preferably at the 
outby end of the portable cable. 

Note: The frame to ground potential shall 
not exceed 40 volts. 

(c) A device(s) such as a diode(s) of 
adequate peak inverse voltage rating 
and current-carrying capacity to con- 
duct possible fault current through 
the grounded power conductor. Diode 
installations shall include: (1) An over- 
current device in series with the diode, 
the contacts of which are in the ma- 
chine's control circuit; and (2) a block- 
ing diode in the control circuit to pre- 
vent operation of the machine with 
the polarity reversed. 

S IS-.S! Electrical protection of circuits 
and equipment. 

(a) An automatic circuit-interrupting 
device(s) shall be used to protect each 
ungrounded conductor of a branch cir- 
cuit at the junction with the main cir- 
cuit when the branch-circuit 
conductor(s) has a current carrying ca- 
pacity less than 50 percent of the main 
circuit conductor(s), unless the protec- 
tive device(s) in the main circuit will 
also provide adequate protection for 
the branch circuit. The setting of each 
device shall be specified. For headlight 



and control circuits, each conductor 
shall be protected by a fuse or equiva- 
lent. Any circuit that is entirely con- 
tained in an explosion-proof enclosure 
shall be exempt from these require- 
ments. 

(b) Each motor shall be protected by 
an automatic overcurrent device. One 
protective device will be acceptable 
when two motors of the same rating 
operate simultaneously and perform 
virtually the same duty. 

(1) If the overcurrent-protective 
device in a direct-current circuit does 
not open both lines, particular atten- 
tion shall be given to marking the po- 
larity at the terminals or otherwise 
preventing the possibility of reversing 
connections which would result in 
changing the circuit interrupter to the 
grounded line. 

(2) Three-phase alternating-current 
motors shall have an overcurrent-pro- 
tective device in at least two phases 
such that actuation of a device in one 
phase will cause the opening of all 
three phases. 

(c) Circuit-interrupting devices shall 
be so designed that they can be reset 
without opening the compartment in 
which they are enclosed. 

(d) All magnetic circuit-interrupting 
devices shall be mounted in a manner 
to preclude the possibility of their 
closing by gravity. 

S 18.52 Renewal of fuses. 

Enclosure covers that provide access 
to fuses, other than headlight, control- 
circuit, and handheld-tool fuses, shall 
be interlocked with a circuit-interrupt- 
ing device. Puses shall be inserted on 
the load side of the circuit interrupter. 



Subpart C — Inspections and Tests 

§ 18.60 Detailed inspection of components. 

An inspection of each electrical com- 
ponent shall include the following: 

(a) A detailed check of parts against 
the drawings submitted by the appli- 
cant to determine that: (1) The parts 
and drawings coincide; and (2) the 
minimum requirements stated in this 
part have been met with respect to 
materials, dimensions, configuration, 
workmanship, and adequacy of draw- 
ings and specifications. 

(b) Exact measurement of joints, 
journal bearings, and other flame-ar- 
resting paths. 

(c) Examination for unnecessary 
through holes. 

(d) Examination for adequacy of 
lead-entrance design and construction. 

(e) Examination for adequacy of 
electrical insulation and clearances be- 
tween live parts and between live parts 
and the enclosure. 

(f) Examination for weaknesses in 
welds and flaws in castings. 



52 



(g) Examination for distortion of en- 
closures before tests. 

(h) Examination for adequacy of fas- 
tenings, including size, spacing, secu- 
rity, and possibility of bottoming. 

§ 18.61 Final inspection of complete ma- 
chine. 

(a) A completely assembled new ma- 
chine or a substantially modified 
design of a previously approved one 
shall be inspected by a qualified 
representative(s) of MSHA. When 
such inspection discloses any unsafe 
condition or any feature not in strict 
conformance with the requirements of 
this part it shall be corrected before 
an approval of the machine will be 
issued. A final inspection will be con- 
ducted at the site of manufacture, re- 
building, or other locations at the 
option of MSHA. 

(b) Complete machines shall be in- 
spected for: 

(1) Compliance with the require- 
ments of this part with respect to 
joints, lead entrances, and other perti- 
nent features. 

(2) Wiring between components, ade- 
quacy of mechanical protection for 
cables, adequacy of clamping of cables, 
positioning of cables, particularly with 
respect to proximity to hydraulic com- 
ponents. 

(3) Adequacy of protection against 
damage to headlights, push buttons, 
and any other vulnerable component. 

(4) Settings of overload- and short- 
circuit protective devices. 

(5) Adequacy of means for connect- 
ing and protecting portable cable. 

§ 18.62 Tests to determine explosion-proof 
characteristics. 

(a) In testing for explosion-proof 
characteristics of an enclosure, it shall 
be filled and surrounded with various 
explosive mixtures of natural gas and 
air. The explosive mixture within the 
enclosure will be ignited electrically 
and the explosion pressure developed 
therefrom recorded. The point of igni- 
tion within the enclosure will be 
varied. Motor armatures and/or rotors 
will be stationary in some tests and re- 
volving in others. Coal dust, produced 
by grinding coal from the Pittsburgh 
coal bed to a fineness of minus 200 
mesh, will be added to the explosive 
gas-air mixtures in some tests. At 
MSHA's discretion dummies may be 
substituted for internal electrical com- 
ponents during some of the tests. Not 
less than 16 explosion tests shall be 
conducted: however, the nature of the 
enclosure and the results obtained 
during the tests will determine wheth- 
er additional tests shall be made. 

(b) Explosion tests of an enclosure 
shall not result in: 

( 1 ) Discharge of flame. 



(2) Ignition of an explosive mixture 
surrounding the enclosure. 

(3) Development of afterburning. 

(4) Rupture of any part of the enclo- 
sure or any panel or divider within the 
enclosure. 

(5) Permanent distortion of the en- 
closure exceeding 0.040 inch per linear 
foot. 

(c) When a pressure exceeding 125 
pounds per square inch (gage) is devel- 
oped during explosion tests, MSHA re- 
serves the right to reject an 
enclosure(s) unless (1) constructional 
changes are made that result in a re- 
duction of pressure to 125 pounds per 
square inch (gage) or less, or (2) the 
enclosure withstands a dynamic pres- 
sure of twice the highest value record- 
ed in the initial test. 

• • • 

§ 18.67 Static-pressure tests. 

Static-pressure tests shall be con- 
ducted by the applicant on each enclo- 
sure of a specific design when MSHA 
determines that visual inspection will 
not reveal defects in castings or in 
single-seam welds. Such test procedure 
shall be submitted to MSHA for ap- 
proval and the specifications on file 
with MSHA shall include a statement 
assuring that such tests will be con- 
ducted. The static pressure to be ap- 
plied shall be 150 pounds per square 
inch (gage) or one and one-half times 
the maximum pressure recorded in 
MSHA's explosion tests, whichever is 
greater. 

• • • 
§ 18.69 Adequacy tests. 

MSHA reserves the right to conduct 
appropriate test(s) to verify the ade- 
quacy of equipment for its intended 
service. 

Subpart D — Machines Assembled 
With Certified or Explosion-Proof 
Components, Field Modifications of 
Approved Machines, and Permits 
To Use Experimental Equipment 

§ 18.80 Approval of machines assembled 
with certified or explosion-proof com- 
ponents. 

(a) A machine may be a new assem- 
bly, or a machine rebuilt to perform a 
service that is different from the origi- 
nal function, or a machine converted 
from nonpermissible to permissible 
status, or a machine converted from 
direct- to alternating-current power or 
vice versa. Properly identified compo- 
nents that have been investigated and 
accepted for application on approved 
machines will be accepted in lieu of 
certified components. 

(b) A single layout drawing (see 
Figure 1 in Appendix II) or photo- 
graphs will be acceptable to identify a 
machine that was assembled with cer- 



tified or explosion-proof components. 
The following information shall be 
furnished: 

(1) Overall dimensions. 

(2) Wiring diagram. 

(3) List of all components (see 
Figure 2 in Appendix II) identifying 
each according to its certification 
number or the approval number of the 
machine of which the component was 
a part. 

(4) Specifications for: 

(i) Overcurrent protection of motors. 

(ii) All wirjng between components, 
including mechanical protection such 
as hose conduits and clamps. 

(iii) Portable cable, including the 
type, length, outside diameter, and 
number and size of conductors. 

(iv) Insulated strain clamp for ma- 
chine end of portable cable. 

(V) Short-circuit protection to be 
provided at outby end of portable 
cable. 

(c) MSHA reserves the right to in- 
spect and to retest any component(s) 
that had been in previous service, as it 
deems appropriate. 

(d) Fees for testing under this sub- 
part shall be consistent with those 
stated in § 18.7. 

(e) When MSHA has determined 
that all applicable requirements of 
this part have been met, the applicant 
will be authorized to attach an approv- 
al plate to each machine that is built 
in strict accordance with the drawings 
and specifications filed with MSHA 
and listed with MSHA's formal ap- 
proval. A design of the approval plate 
will accompany the notification of ap- 
proval. (Refer to §§ 18.10 and 18.11.) 

(f) Approvals are issued only by Ap- 
proval and Certification Center, Box 
201B Industrial Park Road. Dallas 
Pilie, Triadelphia, W. Va. 26049. 




£5]^....., 



'^'^^-(^ftP 



:,.rE 



53 



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f/Sum 9- 










MUM -^^-^ 






PART 75— MANDATORY SAFETY 
STANDARDS— UNDERGROUND 
COAL MINES 



Subpart I — Underground High-Voltage 
Distribution 

75.800 High-voltage circuits; circuit break- 
ers. 

75.800-1 Circuit breal<ers; location. 

75.800-2 Approved circuit schemes. 

75.800-3 Testing, examination and mainte- 
nance of circuit breakers; procedures. 

75.800-4 Testing, examination, and mainte- 
nance of circuit breakers; record. 

75.801 Grounding resistors. 

75.802 Protection of high-voltage circuits 
extending underground. 

75.803 Fail safe ground check circuits on 
high-voltage resistance grounded s.vs- 
tems. 

75.803-1 Maximum voltage ground check 
circuits. 

75.803-2 Ground check systems not em- 
ploying pilot check wires; approval by 
the Secretary. 

75.804 Underground high-voltage cables. 

75.805 Couplers. 

75.806 Connection of single-phase loads. 

75.807 Installation of high-voltage trans- 
mission cables. 

75.808 Disconnecting devices. 

75.809 Identification of circuit breakers 
and disconnecting switches. 

75.810 High-voltage trailing cables; splices. 

75.811 High-voltage underground equip- 
ment; grounding. 



75.812 Movement of high-voltage power 
centers and portable transformers; 
permit. 

75.812-1 Qualified person, 

75.812-2 High-voltage power centers and 
transformers; record of examination. 



Subpart I — Underground High- 
Voltage Distribution 

§ 75.800 High-voltage circuit.s; circuit 
breakers. 

[Statutory Provisions] 

High-voltage circuits entering the 
underground area of any coal mine 
shall be protected by suitable circuit 
breakers of adequate interrupting ca- 
pacity which are properly tested and 
maintained as prescribed by the Secre- 
tary. Such breakers shall be equipped 
with devices to provide protection 
against under-voltage grounded phase, 
short circuit, and overcurrent. 



§ 75.800-2 Approved circuit schemes. 

The following circuit schemes will be 
regarded as providing the necessary 
protection to the circuits required by 
§ 75.800: 

(a) Ground check relays may be used 
for undervoltage protection if the 
relay coils are designed to trip the cir- 
cuit breaker when line voltage de- 
creases to 40 percent to 60 percent of 
the nominal line voltage; 

(b) Ground trip relays on resistance 
grounded systems will be acceptable as 
grounded phase protection; 

(c) One circuit breaker may be used 
to protect two or more branch circuits, 
if the circuit breaker is adjusted to 
afford overcurrent protection for the 
smallest conductor. 

§ 75.800-3 Testing, examination and main- 
tenance of circuit breakers; procedures. 

(a) Circuit breakers and their auxil- 
iary devices protecting underground 
high-voltage circuits shall be tested 
and examined at least once each 
month by a person qualified as pro- 
vided in § 75.153; 

(b) Tests shall include: 

(1) Breaking continuity of the 
ground check conductor, where 
ground check monitoring is used; and 

(2) Actuating at least two (2) of the 
auxiliary protective relays. 

(c) Examination shall include visual 
observation of all components of the 
circuit breaker and its auxiliary de- 
vices, and such repairs or adjustments 
as are indicated by such tests and ex- 
aminations shall be carried out imme- 
diately. 

§ 7.5.800-4 Testing, examination and main- 
tenance of circuit breakers; record. 



The operator of any coal mine shall 
maintain a written record of each test, 
examination, repair, or adjustment of 
all circuit breakers protecting high 
voltage circuits which enter any un- 
derground area of the coal mine. Such 
record shall be kept in a book ap- 
proved by the Secretary. 

§ 75.801 Grounding resistors. 

[Statutory Provisions] 

The grounding resistor, where re- 
quired, shall be of the proper ohmic 
value to limit the voltage drop in the 
grounding circuit external to the resis- 
tor to not more than 100 volts under 
fault conditions. The grounding resis- 
tor shall be rated for maximum fault 
current continuously and insulated 
from ground for a voltage equal to the 
phase-to-phase voltage of the system. 



S 75.802 Protection of high-voltage cir- 
cuits extending underground. 

(a) Except as provided in paragraph 
(b) of this section, high-voltage cir- 
cuits extending underground and sup- 
plying portable, mobile, or, stationary 
high-voltage equipment shall contain 
either a direct or derived neutral 
which shall be grounded through a 
suitable resistor at the source trans- 
formers, and a grounding circuit, origi- 
nating at the grounded side of the 
grounding resistor, shall extend along 
with the power conductors and serve 
as a grounding conductor for the 
frames of all high-voltage equipment 
supplied power from that circuit. 



S 75.803 Fail safe ground check circuits 
on high-voltage resistance grounded 
systems. 

[Statutory Provisions] 

On and after September 30, 1970, 
high-voltage, resistance grounded sys- 
tems shall include a fail safe ground 
check circuit to monitor continuously 
the grounding circuit to assure con- 
tinuity and the fail safe ground check 
circuit shall cause the circuit breaker 
to open when either the ground or 
pilot check wire is broken, or other no 
less effective device approved by the 
Secretary or his authorized repre- 
sentative to assure such continuity, 
except that an extension of time, not 
in excess of 12 months, may be permit- 
ted by the Secretary on a mine-by- 
mine basis if he determines that such 
equipment is not available. 

§75.803-1 Maximum voltage ground 
check circuits. 

The maximum voltage used for 
ground check circuits under § 75.803 
shall not exceed 96 volts. 



54 



S 75.803-2 Ground check systems not em- 
ploying pilot check wires; approval by 
the Secretary. 

Ground check systems not employ- 
ing pilot check wires will be approved 
only if it is determined that the 
system includes a fail safe design caus- 
ing the circuit breaker to open when 
ground continuity is broken. 



S 75.S04 rnderground high-voltage cables. 

(a) Underground high-voltage cables 
used in resistance grounded systems 
shall be equipped with metallic shields 
around each power conductor with one 
or more ground conductors having a 
total cross sectional area of not less 
than one-half the power conductor, 
and with an insulated external con- 
ductor not smaller than No. 8 
(A.W.G.) or an insulated internal 
ground check conductor not smaller 
than No. 10 (A.W.G.) for the ground 
continuity check circuit. 

(b) All such cables shall be adequate 
for the intended current and voltage. 
Splices made in such cables shall pro- 
vide continuity of all components. 



§75.805 Couplers. 

[Statutory Provisions] 

Couplers that are used with 
medium-voltage or high-voltage power 
circuits shall be of the three-phase 
type with a full metallic shell, except 
that the Secretary may permit, under 
such guidelines as he may prescribe, 
no less effective couplers constructed 
of materials other than metal. Cou- 
plers shall be adequate for the voltage 
and current expected. All exposed 
metal on the metallic couplers shall be 
grounded to the ground conductor in 
the cable. The coupler shall be con- 
structed so that the ground check con- 
tinuity conductor shall be broken first 
and the ground conductors shall be 
broken last when the coupler is being 
uncoupled. 

• • • 
§ 75.808 Disconnecting devices. 

[Statutory Provisions] 

Disconnecting devices shall be in- 
stalled at the beginning of branch 
lines in high-voltage circuits and 
equipped or designed in such a manner 



that it can be determined by visual ob- 
servation that the circuit is deener- 
gized when the switches are open. 

• • • 

S 75.810 High-voltage trailing cable.s; splices. 
[Statutory Provisions] 

In the case of high-voltage cables 
used as trailing cables, temporary 
splices shall not be used and all per- 
manent splices shall be made in ac- 
cordance with § 75.604. Terminations 
and splices in all other high-voltage 
cables shall be made in accordance 
with the manufacturer's specifica- 
tions. 

S 75.81 1 High-voltage underground equip- 
ment; grounding. 

[Statutory Provisions] 

Frames, supporting structures and 
enclosures of stationary, portable, or 
mobile underground high-voltage 
equipment and all high-voltage equip- 
ment supplying power to such equip- 
ment receiving power from resistance 
grounded systems shall be effectively 
grounded to the high-voltage ground. 

• • • 



55 



DEMONSTRATION OF THE DISCRIMINATING CIRCUIT BREAKER (DISCB) 
By Michael R. Yenchekl 



ABSTRACT 



The evolution of the DISCB concept and 
theory of operation are described brief- 
ly. Laboratory test results with a simu- 
lated mine haulageway are included and 
illustrate detector operation, and the 
effects of rectifier ripple, arcing, and 



deteriorating track bonding. Future 
Federal Bureau of Mines laboratory and 
fieldwork plans are outlined in conclu- 
sion along with an appendix containing 
important points for consideration during 
in-mine installation. 



INTRODUCTION 



Track haulage systems in United States 
underground coal mines operate at 300 to 
600 V dc, one side of which returns to 
the source through grounded rails. Elec- 
trical faults on these systems are a 
major cause of mine fires, and once hav- 
ing caused a fire, can also block egress 
from the mine and contaminate the fresh 
air supply. 

From 1952 to 1977, Federal personnel 
investigated 127 such fires. At least 
80 would have been prevented if 



suitable electrical 
available. 



protection had been 



The simple overcurrent sensing devices 
commonly used in haulage systems date 
back to the 1920' s despite advances in 
electrical and electronic technology. 
What is needed is a protection scheme 
that permits the flow of thousands of 
amperes of normal motor currents, but 
responds rapidly to the low-level ground 
fault currents associated with incendiary 
arcing. 



THE DISCB CONCEPT 



In the early 1960's, French researchers 
(8^)2 successfully developed a scheme for 
accon^jlishing the required discrimination 
by impressing an audio frequency tone on 
the trolley line at each rectifier sta- 
tion and monitoring its magnitude. The 
need for modification of the system, to 
accommodate the heavier rolling stock 
prevalant in U. S. mines, led to Bureau 
of Mines research contract HO 122058 with 
Westinghouse Electric Corp. in 1972. 

Through the DISCB concept, arcing and 
other types of faults are detected as 
illegitimate loads because of the low 
in5)edance they present to 3-kHz-ac cur- 
rent. This frequency was chosen because 

'Electrical engineer, Pittsburgh Re- 
search Center, Bureau of Mines, Pitts- 
burgh, Pa. 

^Underlined numbers in parentheses 
refer to items in the list of references 
preceding the appendix. 



it gives good signal transmission on un- 
derground trolley wires, yet is high 
enough to permit a clean separation of 
the signal from normal system noise. 

Many small mobile loads such as jeeps 
have sufficient motor inductance to pre- 
vent significant 3-kHz current from flow- 
ing, however, larger haulage locomotives 
must be equipped with filters to raise 
their impedance at 3 kHz as shown in fig- 
ure 1. Applying this technique, Westing- 
house ( 13 ) found it possible to detect 
illegitimate impedances of 20 fi or less, 
corresponding to fault currents of 15 A 
or more on a 300-V system. However, dur- 
ing underground tests the filtering de- 
vices needed on most mine vehicles pre- 
sented a problem. This equipment had to 
be mounted in exposed areas and was sub- 
jected to severe mechanical stress. 
Because of this a simplified method was 
sought that significantly reduced the 
number of filters needed. 



56 



Trolley and feeder wire 




FIGURE 1. = DISCB current flow. 



3-kHz 

oscillator 

with 

inby and outby 

controls 



,_l(_frrJLnnrL|^, 

r Outby Inby^ 



i 



dc 
substation 



-Lightweight cable strung near the 
trolley wire -opened to trip contactors 
and remove trolley power 



3-kHz 
current 
detector 



-0-^ 



3-kHz 
voltage 
detector 



3-kHz 
current 
detector 



3-kHz 

oscillator 

with 

inby and outby 

controls 



,^^jTnJLnnrL|(^, 

A Outby 



Inby 



i 



dc 
substation 



FIGURE 2, - Simplified DISCB system. 



57 



Work by Mine Safety and Health Adminis- 
tration (MSHA) personnel (4^) indicated 
that arcing faults on 300-V trolley sys- 
tems are much less likely to be sustained 
at current levels smaller than 200 A. 
Below this limit arc voltage increases 
rapidly and the arc easily extinguishes 
itself. Thus a discriminating circuit 
breaker system capable of detecting any 
faults in excess of 150 A will provide 
substantial protection. 

In the final modified design, 3-kHz-ac 
current detectors are used to detect 
illegitimate loads up to 1,200 ft from 
the substation. For more remote faults 
the inductance of the trolley wire pre- 
vents adequate high frequency current 
from flowing; however, the same induc- 
tance causes a substantial high frequency 
voltage drop which can be recognized by a 
3-kHz-ac undervoltage detector remote 
from the substation (see fig. 2). This 
combination of voltage and current detec- 
tors provides detection of any illegiti- 
mate load in excess of 150 A, without 
recourse to filters on other than the 
largest vehicles. 

In operation a lightweight pilot wire 
alongside the trolley wire carries sig- 
nals to coordinate the operation of dc 
breakers at the various sources to 
interrupt all lines feeding the fault. 
This wire also provides additional pro- 
tection, in that if the cable is broken 
by a roof fall, the dc power is inter- 
rupted and cannot be energized until 
repairs have been made. 

System Benefits 

If those fault conditions on coal mine 
track haulageways which are usually not 
reported are considered, it is found that 
there is a probable continuous benefit to 
be derived from discriminating circuit 
breakers. Not all short circuits start 
fires but they often stress and damage 
equipment. This damage can be signifi- 
cantly reduced with the quick response of 
the discriminating circuit breakers. 



Inquiries within the industry indicate 
that such incidents may occur about two 
to five times a year. Inby production 
stops for an average of 4 hours while 
repairs are made. If a 150-worker-per 
shift mine with one-third the workers 
idled by the outage is assumed, and 
equating 1 man-hour of labor to 1 ton of 
coal at $40 per ton, the annual worth of 
a trolley wire protection scheme is esti- 
mated as 2 to 5 (mishaps) x 4 (hours to 
repairs) x 50 (workers) x 40 ($ man-hour) 
= $16,000 to $40,000. 

If the initial cost of a discriminating 
circuit breaker system is estimated to be 
in the range of $35,000 to $70,000, the 
payback time is of the order of 2 years, 
an acceptable period. Thus trolley wire 
protection appears justified on economic 
grounds alone ( 11 , p. 12). 

Of all the protection schemes proposed 
to date, the DISCB offers the best hope 
of functioning well, given proper in- 
stallation. It does not depend on uncon- 
trolled characteristics such as rectifier 
ripple, transient waveforms, or dl/dt 
level sensing for its basic operation. 
Also, the DISCB can be employed on any 
existing haulageway with minimal modifi- 
cations to the haulage equipment. 
Finally, it utilizes low-power solid- 
state electronics (fig. 3) that can give 
virtually maintenance-free performance 
for many years. 

After the development of the discrimin- 
ating circuit breaker, a system was in- 
stalled underground and exposed to typ- 
ical haulage conditions including 
electrical power system fluctuations for 
over 5 years (see fig. 4). It performed 
satisfactorily but operated event coun- 
ters in lieu of tripping circuit break- 
ers. What remains to be demonstrated is 
that the system, in the long term, will 
work reliably and safely when actually 
protecting a mine haulageway. The appen- 
dix to this paper provides recommenda- 
tions for field installation. 



58 



■zmm^/^. 




FIGURE 3, "> DISCB internal components, 




FIGURE 4, - DISCB control installed underground. 



59 



HAULAGEWAY MODEL 



Background 

The management of Federal No. 1 Mine of 
Eastern Associated Coal Co. , Grant Town, 
W. Va. , expressed an interest in utiliz- 
ing the system to protect a 1-mile sec- 
tion in the oldest but still actively 
used area of the mine. Prior to commit- 
ment they requested a laboratory demon- 
stration of the DISCB basic functions 
using prototype hardware and a simulation 
of the particular haulage section. The 
Bureau of Mines, therefore, has recently 
constructed and successsfully operated a 
lumped parameter simulation of the rail 
section, protected by the actual DISCB 
equipment. 



empties, connect the rotary dump area 
with the active sections of the mine. At 
the No. 1 substation a 500-kW mercury arc 
hewittic rectifier was tied into the 
system through a circuit breaker having 
an overcurrent setting of 2,500 A. It 
has since been replaced with a solid- 
state unit. 

The positive No. 9 section copper 
trolley is paralleled part of the way by 
a 1,590 kcmil aluminum feeder cable, tied 
to the trolley at 200-ft intervals. The 
track conductors consist of 85-lb double- 
bonded rails. The distance between 
trolley and feeder is 12 in; between 
trolley and rail it averages 72 in. 



Federal Haulageway 

The Federal No. 1 Mine was visited to 
gather data on a portion of the rail 
haulage fed from a single 300-V source 
shown in figure 5. Two parallel track 
entries, one for loads and the other for 



The available locomotive loads are: 
Two 50-ton locomotives with four 160-hp 
motors, six 37-ton locomotives with four 
120-hp motors, and two 15-ton locomotives 
with two 150-hp motors. Numerous utility 
vehicles of 150 hp and less are also 
used. 



^ 



P^^ 



\^ 



FIGURE 5. - Portion of Federal No. 1 haulage 
used for model. 



Theoretical Analysis 

The rectifier can be represented by the 
equivalent circuit shown in figure 6. 

Mine rectifiers generally are found in 
one of two configurations: The three- 
phase bridge and the six-phase double wye 
(12). It can be shown that the operation 
of both of these circuits is equivalent 
(14) . The steady-state regulation curve 
of either circuit is shown in figure 7. 

The effective source resistance, V/I, 
is not constant but is lower in the over- 
load range than for the short circuit. 
The source resistance, Rg, may be calcu- 
lated given the per-unit reactance and 
resistance of the transformer rectifier. 
For a 500-kW unit, typically percent R 
equals 1.1, percent X equals 7.5, and 
percent Z equals 7.6. 



60 



Overload 
range 



^SOURCE 



AAAAr 



1-SOURCE 

-onrvnn — 




FIGURE 6. - Direct current mine power supply. 



Short circuit 
FIGURE 7. - Rectifier voltage regulation. 



^— I 



Assuming an infinitely stiff source feeding a 500-kW three-phase bridge rectifier, 
the ac impedance can be calculated as (3, pp. 12-17) 



300 



= 128 V, 



and 

Therefore, 



and 



•LINE - NEUTRAL I.35/3 1.35/3 

Iline = 0*816 Idc = 0'816 (1,666) = 1,360 A, 
ZgASE = 128/1,360 = 0.094 ^. 

Rac = (0.11)(0.094 n) = 1.03 mn, 
Z^Q = (0.076)(0.094 fi) = 7.14 mfi, 
Xac = (0.075)(0.094 fl) = 7.05 mfi, 

X/377 



7.05 (1C"3) 
377 



'AC ^ ^/~>'/ TT=7 18*7 yH, 

For the overload range the equivalent dc circuit impedance is ( 14 ) 

^SOURCE = 6 fL;^c + 2 R^C = (360)(18. 7)10-6 + 2( 1 .03) (10-3) 
= 8.79 ma. 
For the short-circuit case 

RSOURCE = ^ ^AC= /3 (7.14)(10-3) 
= 12.37 mfi 
The equivalent source inductance is essentially constant and equal to (14) 



'SOURCE 



1.65 L.p = 1.65(18.7)10-6 = 31 ^h. 



Since the DISCB detects relatively low levels of fault current, the equivalent source 
resistance for the overload range was chosen for the model. 



The theoretical dc resistance at 20° C 
for 400 kcmil, figure 9 hard-drawn copper 
trolley wire is (1) 0.02687 n/1,000 ft. 
For the 1,590 kcmil aluminum feeder it is 
(J^) 0.01091 fi/1,000 ft, or roughly 
equivalent to 1,000 kcmil copper. So the 
paralleled trolley and feeder resistance 
is 0.00755 n/1,000 ft 

The resistance of two 85-lb rails 
cross-bonded at 200-ft intervals and hav- 
ing 33 bonded joints per rail per 1,000 
ft is (6^) 0.0064 fl/1,000 ft. 

Actual measurements (9^) of unbonded 
joints indicate that their resistance 
averages 50 times that of a well-bonded 
joint. Resistances of unbonded 85-lb 
rail joints have been measured (2^) to be 
0.025 Sl» In simulating poor bonding 
for a pair of 85-lb rail it is assumed 
that 70 pet of the joints are unbonded. 
Thus the dc resistance becomes 0.335 
fi/1,000 ft. 

Because the DISCB imposes a 3-kHz sig- 
nal directly onto the haulage system con- 
ductors, the importance of skin effect 
was considered. Let R' be the effective 
ac resistance for a linear cylinderical 
conductor and R the dc resistance; then 



the aluminum feeder x = 14.92, k = 5.53, 
so ac resistance is 0.0637 fi/1,000 ft 
and, for trolley and feeder is parallel, 



R 



kR, 



where k can be determined from standard 
references O, p. 4-29) in terms of 

X = 0.0636 /^ 

where f = frequency in hertz, 

\i = magnetic primability of the 
conductor (assumed constant), 

and R = dc resistance at 20° C. 



3kHz 



0.0373 fl/1,000. 



For steel rails the value of y, and 
thus R' , will vary and should be deter- 
mined by test. Measured ( 16 ) values of 
ac resistance versus current indi- 
cate that between 500 and 800 A, R' is 
almost constant and a maximum. As this 
range is of interest for the DISCB, an 
approximate extrapolation of the curves 
yielded 

R'3kH2 = 0.3273 J^/1,000 ft 

for 85-lb double-bonded track. 

The inductance of any trolley system 
configuration may be calculated theoret- 
ically by several methods (2_, _7) with the 
following assumptions: 

1. All conductors are nonmagnetic. 

2. All conductors are cylindrical. 

3. Constant spacing exists between 
conductors. 

4. Rail self -inductance is negligible. 

5. The cross-sectional area of feeder 
is added to trolley and/or rails. 

Accurate field measurements of system 
inductance yields results in substantial 
agreement with the theoretical values. 
Therefore, it was not considered neces- 
sary to choose inductance values for the 
haulage model based upon rigorous theo- 
retical calculations; instead, they are 
reasonable estimates from field surveys 
(_5, pp. 9-1, 9-13) of systems similar to 
Federal No. 1. Thus 



For the 9-section copper trolley at 
3,000 Hz. 



= 0.0636 M^^ . 9.25. K = 3, 
/ 0.142 



60, 



so the resistance of the trolley to a 3- 
kHz voltage is 0.09734 fi/1,000 ft. For 



L9s&85# - 0'^ mH/1,000 ft, Xl 
= 9.3 fl/1,000 ft at 3 kHz 
and L9s|JAi&85# " ^.3 mH/1,000 ft, Xl 
= 5.7 J2/l,000 ft at 3 kHz. 



62 



In general, the use of parallel feeder 
conductors decreases inductance while 
greater conductor separation increases 
it. 

The shunt capacitance between the sys- 
tem conductors can be determined by indi- 
vidually calculating capacitance to neu- 
tral points and combining the resultant 
values in series and parallel as neces- 
sary. The equation that is used is ( 15 , 
pp. 77-83) 



0.0388 



log (Di/R;) 



pf /mile, 



vyw^ 



FIGURES. 



vwv 

22 ii 
(Includes lights) 

Electrical model of mine haulage 
locomotive. 



where ^fi - the capacitance of a conduc- 
tor to a neutral point, 

R j = the radius or equivalent 
radius of the conductor. 



such as pumps and lights distributed 
along the haulage were simulated using 90 
fi per 500 ft. 

Construction of the Model 



and 



Dj = the distance to the neutral 
point between conductors. 



The values arrived at by these calcula- 
tions are, for the trolley and or feeder 
and track, Cfg equals 0.016 pF/1,000 ft; 
and for the trolley and track, C^ equals 
0.005 pF/1,000 ft. The respective shunt 
capacitive reactances at 3 kHz are X_ 



The actual haulage system routing was 
rearranged, as shown in figure 9, to fit 
on a 4- by 8-ft plywood board. It was 
subdivided into sections and simulated as 
shown in figure 10 where L is the system 
inductance per section length. The par- 



allel combination of R 



AC 



and 



^DC 



in 



series with R simulates dc resistance, 
and R/\Q + R, the ac resistance; L< 



is 



equals 3.3 kn/1,000 ft and X^, equals 10.6 sufficiently large to approximate skin 



kj^/1,000 ft. For modeling purposes the 
shunt capacitance was neglected. 

Large mobile haulage loads on dc mine 
systems utilize series field dc motors. 
Empirical relationships for 300-V-dc 
motors show that the effective inductance 
can be approximated by ( 12 , pp. 4-18) 

La = 190/hp rating (mH). 

The circuit simulation is shown in figure 
8. The starting resistance, R^, can be 
varied to produce up to triple full load 
current. Stationary loads (11, p. 16) 



effect at 3 kHz, 



represents distrib- 



uted stationary loading and R3, the high 
resistance of poor bonding (normally 
jumpered) . 



Owing to power 
the lab, loading 
did not exceed 100 
copper magnet wire 
to form inductors, 
ance values were 



source limitations in 
and fault simulations 
A dc. Number 8 square 
was wound on the lathe 
Appropriate resist- 
obtained with nickel- 



chromium wire noninductively wound. 

The demonstration board is shown in 
figure 11. 



63 




LEGEND 

= 9S trolley II 1.5 MCM 
aluminum feeder 

9S trolley 

^-*^ 1.5 MCM aluminum feeder 

++++• 85Hb double bonded track 

-O- Rectifier No. I, Hewittic 
300 V, 500 kW 



^^ ITE circuit breaker 
ra Dump 

FIGURE 9. - Federal No. 1 haulage model. 



/ 1 I I I I I I I I I I I I I I I I I I I I I M I I I I I 



+ »■ 



— •- 



Rdc Lsk 



AAAAr 



^ac 



VNAA/^ 



Rs 




FIGURE 10. - Lumped haulage simulation. 



64 







z 



"H 



'*^4r.;;^##H%i^ *****^4**44##..ff#i*#*jH4w#^l^ 






.s mzm Miummum *tf©ft 



*tcrjr»f« ^ tm 



MfWITTIC^ 30© y^ S©0 KH 



iTf cmcyif »ii«AKfii 




FIGURE 11, - Haulageway model. 



LAB DEMONSTRATION 



Current and Voltage Detection 

Upon completion of the model the dis- 
criminating circuit breaker controls were 
connected to impress the 3-kHz signal on 
the system at the rectifier location as 
shown in figures 12 and 13. The 3-kHz 
current flow with no external mobile load 
or faults connected was 1.17 A as mea- 
sured by the current detector. Referring 
to figure 9, with a 1.5-fi resistive fault 
at point B, the rectifier, the 3-kHz cur- 
rent increases to 4.42 A: the current 



detector relay is activated and the cir- 
cuit breaker trips. A simulated 15-ton 
locomotive placed at B drew 2.30 A at 3 
kHz and did not trip the breaker. 

Applying the fault at point A, 3,450 ft 
from the source, the total high frequency 
current increases slightly over the no- 
load value, to 1.24 A. This point is 
past the protective range of the current 
detector where audio current magnitude 
remains relatively unchanged for resis- 
tive faults remote from the substation. 



65 




FIGURE 12. - Laboratory setup. 



It is here that the DISCB voltage under normal, abnormal, and no-load con- 



detector is needed and a simple exam- 
ple will illustrate this. Referring to 
table 1 the high frequency voltage was 
monitored (fig. 14) at six locations 



ditions. 


Location 


while A, 


G, U, T, 


it. 





B is at the substation 
and Q are remote from 



TABLE I. 



3 kHz voltage variations 



Load condition 


Location 




B 


A 


G 


u 


T 


Q 


No-load 


8.0 
6.1 
7.1 
7.8 
7.9 
7.9 
7.9 


6.7 
5.0 
5.9 
.2 
1.4 
6.6 
6.6 


6.3 
4.8 
5.7 
6.1 
6.1 
.1 
1.3 


6.8 
5.2 
6.0 
6.6 
6.7 
6.7 
6.7 


6.9 
5.3 
6.1 
6.7 
6.8 
6.8 
6.8 


7.1 


1 . 5-J2 fault at B 


5.4 


15-ton locomotive at B.... 
1.5-J^ fault at A 


6.3 
6.9 


15-ton locomotive at A.... 
1,5—0 fault at G.... 


7.0 
7.0 


15-ton locomotive at G. . . . 


7.0 



66 




FIGURE 13. - DISCB controls at substation. 



67 



^.-n T'l 




FIGURE 14, - Voltage measurements on model. 



No-load is defined as that time when 
only distributed stationary loads such 
as pumps and lights are connected on the 
system. The high frequency voltage is a 
maximum at the rectifier and drops by 
22 pet at the remotest point. With a 
fault near the rectifier the 3-kHz volt- 
age throughout the system decreases 24 
pet from the no-load value. The voltages 
at B for a fault or a locomotive differ 
by 15 pet. Since this margin between 
legitimate and illegitimate loads is in- 
sufficient for discrimination a voltage 



detector located near the 
no purpose. 



source serves 



Away from the substation, high current 
loads and faults substantially alter the 
3-kHz voltage distribution. With the 
fault at A the signal voltage there 
drops to 3 pet of the no-load value. It 
also drops substantially with a legiti- 
mate locomotive load there. However, now 
there is an 86-pct difference in the two 
voltages, large enough to adjust the set- 
ting of the voltage detector to protect 



68 



against resistive faults. It is of in- 
terest to note that the voltage magnitude 
remains relatively unchanged at locations 
remote from the fault and the rectifier. 

DISCB worst-case performance is illus- 
trated in figure 15 with a voltage detec- 
tor located 2,875 ft away from the recti- 
fier at A. Through judicious placement 
of the voltage detectors it is possible 
to protect the entire system. 

Active Impedance Multiplier 

As described in the first section, the 
3-kHz impedance of vehicles rated 25 tons 
and larger must be raised sufficiently to 
prevent nuisance tripping. This is 
accomplished by mounting an active imped- 
ance multiplier (fig. 16) on board large 
mobile loads. Laboratory testing of the 
multiplier with a simulated 37-ton loco- 
motive yielded satisfactory results. 



\ ' 


1 1 


_ 


^\s\ ^-Current due to 
- VX/ 1-5 a fault 




- 


- \\\ 


Location of 
voltage detector 


'V^ 


Voltage due to 

15-ton 
. locomotive^ 


Jrip . 
__,^levels 


Current due to \~ " 
]5-ton locomotive n. 


-____~------.^ 


^V/ 


Voltage due to 
"^,^_^-l.5ii fault 


/ 


Current 
detector 




^y/ 


detector 





600 1,200 1,800 2,400 3,00C 

DISTANCE FROM RECTIFIER, ft 

FIGURE 15. - DISCB protection. 




f 




FIGURE 16. = Active impedance multiplier (AIM) v^^ith power supply. 



69 



Signal currents and voltages were mea- 
sured with the load at the rectifier. 
Using the multiplier the current drawn 
was 1.3 A. Without it current increased 
to 2.5 A. The voltage at remote points 
remained unchanged. 

Moving the locomotive to point A the 
current level was not changed by the mul- 
tiplier's exclusion. However, the volt- 
age decreased from 5.0 to 1.0 V. Figure 
17 illustrates the effect graphically. 

Poor Track Bonding 



Effects of Arcing 

A series of arcing fault tests were 
conducted to note any effect on DISCB 
operation. A resistive fault was applied 
at G in series with two steel electrodes, 
0.5 inch in diameter and separated by an 
air gap. Arcing was initiated by bridg- 
ing the gap with several strands of a 19- 
strand No. 12 AWG wire that vaporized 
upon energization. The air gap was var- 
ied from 3/32 to 5/8 in. The presence of 
the arc did not affect the flow of 3-kHz 
current or DISCB operation. 



Poorly maintained or disconnected track 
bonds will insert an additional impedance 
in the rail circuit and slightly reduce 
the 3-kHz voltage measured at remote 
points. For example, with a poorly 
bonded track simulated between the recti- 
fier and G, and the 15-ton locomotive at 
G, there was a 10-pct reduction in the 
signal voltage at G over the good bonding 
value. 




600 1,200 1,800 2,400 3,000 3,600 
DISTANCE FROM RECTIFIER, ft 

FIGURE 17. - Effects of active impedance mul- 
tiplier (AIM). 



Rectified Versus Generated Input 

The DISCB and the demonstration board 
have been used with both a 30-kW genera- 
tor and a 200-kW rectifier. No differ- 
ence in operation could be detected. 
Satisfactory operation was obtained for 
input voltage fluctuations from 200 to 
350 V dc. 

Further Study 

At present sufficient hardware is 
available in prototype form for small- 
scale demonstrations to interested coal 
operators or for consideration by a manu- 
facturer as a marketable product. 

It is intended to install the system at 
the Federal No. 1 Mine on the portion of 
rail haulage modeled in the laboratory. 
Technical advice will be furnished by the 
Bureau as required throughout the in- 
stallation and initial demonstration 
phases of the single-section system. The 
equipment will remain installed for a 
sufficient time to accumulate an extended 
performance history. The Bureau intent 
is to show that the unit can be operated 
for a 3-month period with no more than 
one nuisance interruption and no instance 
of any failure permitting the trolley 
line to remain energized for a sustained 
ground fault greater than 200 A. 

Typical trolley haulage systems in coal 
mines are powered by multiple dc sources, 
typically about 1 mile apart. The 3-kHz 
DISCB signal is impressed upon the system 



70 



at these substations through an oscilla- 
tor and power amplifier. Since the sig- 
nal can be applied at several separate 
locations, means is provided to minimize 
circulating audio frequency currents by 
selection of a master frequency and 
phase. The power amplifier contains a 
synchronizing unit that locks onto the 
nearest outby oscillator and disengages 
its own master oscillator. If for any 
reason the outermost master oscillator 
controlling the system is unavailable the 
next outby oscillator automatically takes 
over the master role and sets the fre- 
quency and phase of the 3-kHz voltages. 
By this means the integrity of the dis- 
criminating system is maintained even 
when several substations are out of com- 
mission. It is this interaction of DISCB 
power source controls that remains to be 
demonstrated in the Bureau's laboratory 
with a multisource system. 



strung alongside the trolley wire to 
carry signals for the system. For the 
substation breaker to close, proper data 
must be received through the cable. For 
example if the cable is broken by a roof 
fall the dc power cannot be energized. 
Also, if the detector units indicate a 
faulty condition, both inby and outby 
breakers are prevented from closing. The 
pilot wire carries signals to synchronize 
the master oscillators and provides the 
power to operate relays contained in the 
voltage detectors. Finally, it can be 
used to reintroduce the high frequency 
tone onto the trolley at points remote 
from the substation. Thus, the wire 
serves a number of vital functions. How- 
ever, it does require additional labor 
expenditures for installation and mainte- 
nance. So it is desirable to explore 
substitute techniques, such as multiplex- 
ing, to eliminate the pilot wire. 



Upon agreement with a cooperating mine, 
the DISCB system will be installed to 
protect a haulage system having at least 
three branches protected by separate cir- 
cuit breakers and fed from more than one 
dc source. This larger demonstration and 
long-term usage test will prove to the 
mining industry that the system is fail- 
safe, reliable, and effective. 

The present design requires that a 
lightweight cable con5)rising three twist- 
ed pairs of insulated 20 gage wire be 



As the 3-kHz voltages and currents are 
present on the system even when dc power 
is interrupted it is possible to detect 
the location of a fault by walking along 
the wire with ac voltmeter and noting 
where a minimum occurs. It appears fea- 
sible that the fault location can be pin- 
pointed automatically by sampling data 
from the current and voltage detectors. 
Ultimately, this information could be fed 
into a con5)uterized mine monitoring sys- 
tem for readout on the surface. 



REFERENCES 



1. American Society for Testing and 
Materials (Philadelphia, Pa.). Standard 
Specification for Figure 9 Deep-Section 
Grooved and Figure 8 Copper Trolley Wire 
for Industrial Haulage. ASTM Bl 16-64, 
C 7.11, 1965, p. 195. 

2. delong, C. P., and W. L. Cooley. 
Measurement of Rail Bond Impedance. 
Proc. of the Fourth WVU Conf. on 
Coal Mine Electrotechnology , Morgantown, 
W. Va., Aug. 2-4, 1978, pp. 6-1—6-8. 

3. Fink, D. G. , and J. M. Carroll 
(ed. by). Standard Handbook for Elec- 
trical Engineers. McGraw-Hill Book Co., 



Inc., New York, 10th ed. , 1969, pp. 12- 
17, p. 4-29. 

4. Hall, P. M., K. Myers, and W. S. 
Vilchek. Arcing Faults on Direct Cur- 
rent Trolley Systems. Proc. 4th WVU 
Conf. on Coal Mine Electrotechnology, 
Morgantown, W. Va. , Aug. 2-4, 1978, 
pp. 21-1—21-19. 

5. Helfrich, W. , P. M. Hall, and 
R. L. Reynolds. Time Constants of Direct 
Current Trolley Systems. Proc. 5th WVU 
Conf. on Coal Mine Electrotechnology, 
Morgantown, W. Va. , July 30-31, Aug. 1, 
1980, pp. 9-1—13. 



71 



6. Jones, D. C, M. E. Altennis, and 
F. W. Myers. Mechanized Mining Electri- 
cal Applications. The Pennsylvania State 
University, University Park, Pa., 3d ed. , 
1971, p. 211. 

7. Koehler, G. Circuits and Net- 
works. Macmillan Publishing Co., Inc., 
New York, 1955, pp. 198-202. 

8. Laboratorie du Centre D' Etudes et 
Recherches des Charbonnages de France. 
CERCHAR Pub. No. 1306, 1963, 8 pp. 

9. Myers, K. G. Open Rail Bond Re- 
sistance Measurements. MSHA Investiga- 
tive Report C102979, 1979, 4 pp. 

10. Ohio Brass Mining Equipment (Mans- 
field, Ohio). Line Materials. Equipment 
Catalog No. 70, Sec. 100, 1974, p. 1601. 



12. Paice, D. A. , A. B. Shimp, and 
R. P. Putkovich. Circuit Breaker Devel- 
opment and Application. Phase I (Con- 
tract H0122058, Westinghouse Electric 
Corp.). BuMines OFR 103(l)-75, Mar. 12, 
1974, 165 pp.; NTIS PB 248 310. 

13. . Circuit Breaker Develop- 
ment and Application. Phase II (Con- 
tract H0122058, Westinghouse Electric 
Corp.). BuMines OFR 103(2)-75, Mar. 12, 
1974, 75 pp.; NTIS PB 248 311. 



14. Schaeffer, J. 
Theory and Design. 
Inc., New York, 1965, 



Rectifier Circuits 
John Wiley & Sons, 
265 pp. 



15. Stevenson, W. D. , Jr. Elements 
of Power System Analysis. McGraw-Hill 
Book Co., Inc., New York, 3d ed. , 1975, 
pp. 46-61, 77-83. 



11. Paice, D. A. Portable Calibrator 
for DC Circuit Breakers (Contract 
H0122058, Westinghouse Electric Corp.). 
BuMines OFR 73-79, July 1978, 43 pp.; 
NTIS PB 297 732. 



16. Trueblood, H. M., and G. Wanchek. 
Investigation of Rail Impedance. Elec- 
trical Eng. , December 1933, p. 905. 



72 



APPENDIX. —RECOMMENDATIONS FOR FIELD INSTALLATION 



At present, there are no guidelines 
covering the installation of the DISCB in 
an underground mine. Since haulage sys- 
tems vary in size and shape, they must be 
analyzed individually. 

Once a cooperative agreement is reached 
with mine mangagement, an up-to-date mine 
electrical map should be obtained. It 
should show the routing and size of the 
trolley haulage conductors and the loca- 
tions of all power sources and circuit 
breakers. 

Additional design data and special sys- 
tem features can be determined on the 
initial mine visit during a nonproduction 
shift. Incandescent lamp distribution 
and pump locations should be noted as 
well as modifications to enhance trolley 
phone performance such as coiled leads at 
substations or capacitors across dead 
blocks. The number of active impedance 
multipliers needed can be determined by 
tabulating the sizes and horsepower rat- 
ings of the larger locomotives. The 
heavy current welders for bonding rails 
must use inductive resistors to prevent 
nuisance tripping. 



technique ( 12 , pp. 2-21) shown in figure 
A-lA , the oscillator frequency is adjust- 
ed for resonance (V^ is in phase with V^) 
and the values of C, f, V^, and Vg are 
recorded. Another approach O, p. 9-5) 
is shown in figure A-lB. The current is 
recorded upon fault through the test 
resistor. The time constant of the sys- 
tem can be determined by measuring the 
time it takes the current to reach 0.637 
of the peak value. 

By connecting a portable 10-V, 3-kHz 
oscillator and monitoring the current, 
the no-load effects of pumps and lights 
can be measured. A signal voltage dis- 
tribution similar to figure 17 can be 
obtained by taking voltage readings on 
board a small vehicle of about 100 hp as 
it traverses the system. Battery pow- 
ered voltage recorders can be installed 
at key locations underground and left 
running during production time. This 
information is helpful for establishing 
voltage detector protection zones and 
settings. 

Finally, a computer model of the sys- 
tem can reinforce the analysis and can be 



During this initial visit, installation 
details of the DISCB system can be dis- 
cussed. The controls will be located 
nearby the rectifiers so these areas 
should be inspected. The pilot wire can 
be conveniently supported using existing 
communication wire hooks if available. 

Typically, electrical noise on mine 
trolley systems is less than 0.1 V at 3 
kHz ( 11 , p. S).! However, since substan- 
tially higher values occasionally have 
been recorded measurements should be 
made. 

Resistance and inductance can be ap- 
proximated theoretically, given conductor 
size and separation. Underground tests 
can yield more exact values. In the 

Underlined numbers in parentheses refer 
to items in the list of references pre- 
ceding this appendix. 



Trolley wire 



Oscilloscope ( / 





Trolley wire 



5il 



FIGURE A»1. 



Rails 

Testing high frequency 
characteristics. 



73 



updated as the system changes. In this 
manner simultaneous loads and faults can 
be simulated easily. Including the ef- 
fects of distributed stationary loading 



the circuit consists of lumped u sections 
representing 500 ft of trolley wire and 
incorporating nodes for calculation pur- 
poses as shown in figure A-2. 



-orwv 



.i'WW 



_rvw\ 




/WY^. 



J'WW 



FIGURE A-2. - Computer simulation. 



74 



INTERMITTENT DUTY RATING OF TRAILING CABLES 
By George J. Conroy 1 and Herman W. HiH2 



ABSTRACT 



Federal Bureau of Mines sponsored 
research conducted at Penn State Univer- 
sity and West Virginia University has 
resulted in recommendations to the Mine 
Safety and Health Administration (MSHA) 
concerning the maximum current ratings 
for trailing cables used at various duty 
cycles. If approved, the ratings would 



permit smaller size cables than those 
presently required by 30 CFR 18, yet 
would provide equivalent safety when pro- 
tected by circuit breakers which include 
overload trip capabilities. Computer and 
calculator programs for calculating 
allowable ampacity (current capacity) are 
presented. 



INTRODUCTION 



If conductor size for a continuous 
miner cable is chosen on the basis of 
continuous-duty ampacity, the result is a 
very large-diameter, and consequently, 
very heavy trailing cable. This is a 
hardship in the manual handling of the 
trailing cable, particularly as the miner 
backs out to permit cleanup and roof 
bolting. The usual solution has been to 
use as small a cable as the local inspec- 
tor will permit, down to AWG 4/0 or 
smaller, without resorting to any partic- 
ular references or guidelines other than 
a general idea as to what acceptable 
insulation temperature should be. This 
temperature depends on the duty cycle of 
the machine. Consequently, an enforce- 
ment and compliance problem exists in 
that changes in mining pattern or strata, 
or even changing the machine operator, 
can transform a safe cable choice to 
an unsafe one without there being any 
standard available by which the unsafe 
condition can be judged. This paper 
describes a method of determining an 
intermittent-duty ampacity for trail- 
ing cables. The ampacity value can be 

^Supervisory electrical engineer (re- 
tired). Bureau of Mines, Pittsburgh 
Research Center, Pittsburgh, Pa. 

^Assistant Professor of Electrical 
Engineering, West Virginia University, 
Morgantown, W. Va. 



updated with changing mine conditions 
so that dangerous situations can be 
anticipated. 

At the request of MSHA, a 2-year se- 
ries of cable tests were performed by 
the Pennsylvania State University (PSU) 
to study the effects of varying duty 
cycles on cable temperatures and to find 
what modifications of the circuit pro- 
tection devices would be necessary in 
order to maintain safe operation if 
intermittent-duty ratings higher than 
the continuous-duty ampacities were 
permitted. The data from these tests 
were analyzed by both PSU and West Vir- 
ginia University (WVU), and specific 
conclusions have been reached. An 
effective summation of the findings 
concerning circuit protection is to be 
found in the doctoral dissertation of 
George Luxbacher of PSU entitled "Evalu- 
ation of the Effectiveness of Molded- 
Case Circuit Breakers for Trailing-Cable 
Protection, " November 1980. An impor- 
tant conclusion is that by including 
thermal overload elements in the breakers 
it is possible, with proper adjustment, 
to adequately protect the cable despite 
large variations in the duty cycle. 
Please note that the inclusion of these 
elements is essential for safety, if rat- 
ings are based on intermittent duty in a 
situation where the duty cycle can vary. 



75 



METHODS OF CALCULATION 



} 



WVU's analysis of the PSU data, in com- 
bination with previous Bureau research 
on conductors by Derek Paice of Westing- 
house and others, provided reconnnenda- 
tions for sizing conductors, in the form 
of equations, nomographs, and a calcu- 
lator program. A good representation of 
the thermal data, vrLthin the limits of 
experimental error, is given by the 
equation: 



0.13 /A 



(1) 



where 



and 



A = the cross-sectional area of 
the copper conductor, 3 in 
circular mils. 



the thermal 
minutes. 



constant, in 



A single time constant is considered suf- 
ficiently accurate with regard to both 
heating and cooling. 

The relationship which yields a new 
cable rating for int^nnittent duty is 
then 



^int 
duty 

where 



cont 
ratine 



1 - exp (-T,/c) 
1 - exp (-T2/c) 



' (2) 



' , = the total time in minutes 
of a cycle of operation, 

T2 = the operating time in min- 
utes or "on" time within 
a cycle, during which the 
current flows; 

^A convenient relationship, relating 
AWG size to conductor area, is the 
expression: 

A = 105500 exp (-0.232 W) 

where W is the wire size and A is the 
area in circular mils. This holds true 
for all AWG wire sizes with the proviso 
that the larger sizes are represented as 



1/0: 
2/0: 



3/0: 
4/0: 



and 



cont 
rat i n< 



i nt 
duty 



the time constant from 
equation 1, 

the continuous current 
rating, 



the intermittent 
rating. 



duty 



Further explanation is required regard- 
ing the flow of current during a particu- 
lar operating time. It is rare that a 
mining machine would be operated in a 
manner such that current drain would be- 
have as a step function, as shown in fig- 
ure 1. The more usual behavior is as 
seen in figure 2, where many peaks and 
valleys of current drain occur and there 



500 



2.5 5.0 

TIME, min 

FIGURE 1. - Ideal 50 pet duty cycle. 




2 4 

TIME, min 

FIGURE 2. - Realistic current trace for continu- 
ous miner. 



76 



are also multiple levels of even the 
average current, during each cycle. 
Therefore, while the preceding calcula- 
tions yield an intermittent duty rating 
for any given on time, it may not be a 
simple matter to determine whether this 
rating is being exceeded, on the average, 
during the on time. The choice of an on 
time, itself, may not be totally simple, 
as there may be low current drains, such 
as from headlights or idling pump motors, 
throughout the cycle. These small cur- 
rents contribute almost insignificantly 
to the heating of the trailing cable; yet 
their presence con5)licates the defining 
of the operating cycle. Some arbitrary 
ground rules become necessary, such as 
deciding that a single root-mean-square 
(rms) average current will represent the 
drain and that the machine will be con- 
sidered to be on whenever the short-term 
average current exceeds 25 pet of the 
continuous-duty rating of the trailing 
cable. Thus, for a machine powered by an 
AWG 4/0 cable (continuous current rating 
of 180 A) and yielding the current drains 
shown in figure 2, T2 would be 2.25 min 
out of a total cycle time of 5.0 min. 
This would result in a cable current max- 
imum rating of 



A 



int = 180 
duty 



by equation 2. 



exp (-5.0/60) ^ 

exp (-2.25/60) ^^^ ^ 



It is possible to instrumentally deter- 
mine the true rms value of the current. 
However, it is probably adequate for 
present purposes to take the envelope of 
the peak reading at each major step, as 
they appear on an instrument having iner- 
tia in its movement, and estimate an 
"average" value from this. Then 



Note that all of the current drains 
have been included in the average, even 
though the duration of the on time was 
set by ignoring the lowest values of 
drain. 

Comparing this true value with the cal- 
culated rating, it appears that the cable 
is adequate for the observed load. 

A simple assumption for the initial 
calculation of the rating might be to 
agree that intermittent duty will be 
arbitrarily defined for the rating pur- 
pose as some cycle such as 60 min com- 
prised of 50 pet on time, 50 pet off 
time. For the preceding example, this 
would have yielded an intermittent duty 
rating of 252 A. It would still be 
necessary to use the rms averaging tech- 
nique on the actual usage data in order 
to check operation against the calcu- 
lated cable rating. Just what the most 
representative arbitrary cycle might 
be — 50-50, 25-75, etc. — has never been 
decided. 

INSTANTANEOUS TRIP SETTINGS 

Thermal-magnetic trip circuit breakers 
with the thermal overload units sized at 
or below the cable's continuous duty 
ampacity may allow use of higher currents 
with providing adequate protection with 
regard to routine loading at all inter- 
mittent duties. However, nothing in this 
paper should be construed as suggesting 
that the size of fuses or the instanta- 
neous trip setting of circuit breakers 
may be increased above the values per- 
mitted by present regulations for a par- 
ticular size of cable. These values are 
determined by short-circuit considera- 
tions, not by long-term thermal effects. 



z (InTn) 



Z T, 



(3) 



^8002 X 0.05 + 652 X 0.35 + 5752 x 0.20 + 2752 
X 0.85 + 4752 X 0.8 + 152 X 2.75 



5.0 



= 263 A 



by equation 3. 



NOMOGRAPHS 



77 



Figures 3 and 4 conprise a set of nomo- 
graphs that may be used to determine the 
thermal time constant of the cable and 
then, directly, a cable rating factor, 
K, which is equivalent to the entire 
radical expression in equation 2, such 
that 



i nt 
duty 



= K I 



IPCEA* 



(4) 



where I|pcea ^^ '-^^ continuous ampacity 
rated by the Insulated Power Cable Engi- 
neers Association (IPCEA). 



Equation 4 can be used to find cable 
rating factors directly. Nomograph 1 
(fig. 3) is used with the on time, t2, 
and the cable time constant (or the 
cable heating time constant) to deter- 
mine a heating factor, H. The total 
cycle-time, t,, is used with the cable 
time constant (or the cable cooling 
time constant) to determine a cooling 
factor C on the same nomograph. Using 
the values of H and C on nomograph 2 
(fig. 4), one obtains the cable rating 
factor, K. One example is worked out 
on figures 3 and 4 for No. 6 round cable 




Cooling factor 



H 
Heating factor 



Cable rating 
factor 



with 
= 30 



1. 2 — ^ "" " ) 1- 1 — 

min, thus K = 1.97, 



20 min, and time FIGURE 3, - Intermittent duty rating nomograph 1. 



PROGRAMS 



The appendix to this paper gives 
HP41C4 and HP97 programs for calculating 
individual rating values. In addition, 
it gives a BASIC program listing for 



obtaining tabulated intermittent duty 
ampacities applicable to 30 CFR 18 appen- 
dix I rating tables. 



DRAG CABLES 



The rating factors obtained by apply- 
ing the described techniques, with 
30 CFR 18 continuous ratings as a base, 
are not at this time approved by MSHA but 
do represent Bureau of Mines best judg- 
ment on realistic values for cables that 
may or may not be on reels during part 
of the working cycle. If only drag 
cables are considered, the Title 30 con- 
tinuous duty ampacities are probably 

'^Reference to specific programs does 
not imply endorsement by the Bureau of 
Mines. 



excessively conservative, and even higher 
intermit tent -duty ratings for drag cables 
would be possible, if based on revised 
30 CFR 18 tables. This exception is im- 
portant because the greatest need for 
higher ratings may occur as handling dif- 
ficulties are experienced with large- 
diameter drag cables, and it may be pos- 
sible to use smaller cables for the given 
currents. The solution to this problem 
must be in modification of the continuous 
duty tables of 30 CFR 18, not in altering 
the intermittent-duty calculations. 



78 




■f-H \ h 



H h 



4 3 2.5 2 



1.5.1.3 1 .9 0.70.60.50.40.3 0.2 0.1 0.050.01 

C, cooling factor 

/ 

H, heating factor 




Cable 
size 



40 



FIGURE 4. - Intermittent duty rating nomograph 2. 



VOLTAGE DROP 



As the stated calculations are based on 
thermal considerations (that is, maximum 
permitted temperature) and do not take 
into account the effect of the higher 
currents on voltage drop in the cable and 



other circuit elements, it is recommended 
that the trailing cable intermittent-duty 
current not exceed about 250 pet of the 
continuous rating, whatever the calcula- 
tions may give. 



CONCLUSIONS 



This discussion has presented a practi- 
cal method of determining an acceptable 
intermittent-duty loading of trailing 
cables, although further consideration by 



MSHA would be required before ratings 
which result from the calculations could 
be considered as applicable to the coal 
mining industry. 



APPENDIX. —PROGRAMS 



79 



HP41C Program, 



Cable Ampacity Rating Table Buildup — Cycle Time in Minutes, 
Operating Time in Percent 



01 


LBL AMP 


02 


LBL 00 


03 


CYCLE T? 


041 


AVIEW 


051 


STOP 


06 


LBL A 


07 


STO 03 


08 


PCT ON? 


09 


AVIEW 


10 


STOP 


11 


LBL B 


12 


ENTER+ 


13 


.01 


14 


* 


15 


STO 01 


16 


WIRE? 


17 


AVIEW 


18 


STOP 


19 


LBL C 


20 


STO 02 


21 


249 


22 


ENTER+ 


23 


RCL 02 


24 


X<Y? 


25 


GTO c' 


26 


ENTER+ 


27 


1000 


28 


X^Y 


29 


X>Y? 


30 


GTO 05 


31 


* 


32 


GTO 05 



33 


LBL b 


34 


SQRT 


35 


.13 


36 


* 


37 


STO 04 


38 


RATED? 


39 


AVIEW 


40 


STOP 


41 


LBL D 


42 


STO 05 


43 


2.5 


44 


* 


45 


STO 06 


46 


RCL 05 


47 


EXECUTE 


48 


AVIEW 


49 


STOP 


50 


LBL E 


51 


RCL 03 


52 


RCL 01 


53 


* 


54 


STO 09 


55 


FIX 1 


56 


ON T= 


57 


ARCL 09 


58 


AVIEW 


59 


RCL 09 


60 


RCL 04 


61 


/ 


62 


CHS 


63 


EX 


64 


CHS 


65 


1 



66 


+ 


67 


1/X 


68 


ENTERf 


69 


RCL 03 


70 


RCL 04 


71 


/ 


72 


CHS 


73 


EX 


74 


CHS 


75 


1 


76 


+ 


77 


* 


78 


SQRT 


79 


RCL 05 


80 


* 


81 


STO 11 


82 


ENTERf 


83 


RCL 06 


84 


X<Y? 


85 


GTO e 


86 


RCL 11 


87 


GTO a 


88 


LBL e 


89 


RCL 06 


90 


GTO a 


91 


LBL c' 


92 


.232 


93 


* 


94 


CHS 


95 


EX 


96 


105500 


97 


* 



98 


LBL 05 


99 


FIX 


100 


STO 02 


101 


AREA= 


102 


ARCL 02 


103 


AVIEW 


104 


PSE 


105 


GTO b 


106 


LBL a 


107 


STO 07 


108 


FIX 


109 


NEW 1= 


110 


ARCL 07 


111 


AVIEW 


112 


PSE 


113 


PSE 


114 


PSE 


115 


PSE 


116 


NEXT? 


117 


AVIEW 


118 


STOP 


119 


GTO 00 


120 


END 



'■steps 04 and 05 
placed by the single 

^8 through 0, -1, - 
MCM, etc. ) . 



Input Values 

A - Cycle time 
B - Percent on 
C - Wire size^ 
D - 30CFR18 rating 
E - Execute 



and all similar instructions throughout the program may be re- 
instruction, "Prompt". 
2, -3 (representing 2/0, 3/0, 4/0); 250 through 750 (for 250,000 



80 



HP97 Program, Cable Ampacity Rating Table Buildup — Cycle Time In Minutes, 
Operating Time in Percent 



001 *LBLA 

002 PRTX 

003 ST03 

004 R/S 

005 *LBLB 

006 ENT+ 
007 



008 

009 1 

010 X 
Oil DSP2 

012 PRTX 

013 DSPO 

014 STOl 

015 R/S 

016 *LBLC 

017 X=0? 

018 GSB5 

019 PRTX 

020 X 
021 

022 
023 
024 
025 
026 
027 
028 
029 
030 
031 
032 
033 
034 
035 
036 
037 
038 
039 
040 
041 
042 
043 
044 
045 
046 
047 
048 
049 



1 
3 

X 

ST04 
R/S 
*LBLD 
PRTX 
SPC 
ST05 
2 

5 

X 

ST02 
RCL5 
R/S 
*LBLE 
RCL3 
RCLl 

X 

RCL4 

CHS 

eX 

CHS 

1 

+ 

1/X 



^If only AWG 
depress "R/S". 



21 11 

-14 

35 03 

51 

21 12 

-21 

-62 

00 

01 

-35 

-63 02 

-14 

-63 00 

35 01 

51 

21 13 

16-43 

23 05 

-14 

54 

-62 

01 

03 

-35 

35 04 

51 

21 14 

-14 

16-11 

35 05 

02 

-62 

05 

-35 

35 02 

36 05 
51 

21 15 

36 03 

36 01 

-35 

36 04 

-24 

-22 

33 

-22 

01 

-55 

52 

nvunber 



050 
051 
052 
053 
054 
055 
056 
057 
058 
059 
060 
061 
062 
063 
064 
065 
066 
067 
068 
069 
070 
071 



ENT + 
RCL3 
RCL4 

CHS 

eX 
CHS 

1 
+ 

X 

X 
RCL5 

X 

ST06 

ENT + 

RCL2 

X>Y? 

RCL6 

PRTX 

SPC 

SPC 

R/S 



2 

3 

2 

CHS 



072 *LBL5 

073 R/S 
074 

075 
076 
077 
078 
079 
080 
081 
082 
083 
084 
085 
086 
087 
088 
089 



1 

5 
5 



X 

RTN 
R/S 



-21 

36 03 

36 04 

-24 

-22 

33 

-22 

01 

-55 

-35 

54 

36 05 

-35 

35 06 
-21 

36 02 
16-34 
36 06 

-14 

16-11 

16-11 

51 

21 05 

51 

-62 
02 
03 
02 

-22 

-35 
33 
01 
00 
05 
05 
00 
00 

-35 
24 
51 



SAMPLE RUNS 



5-min cycle time 
50 pet on 

(enter 50") 
"0" then 2" 

for AWG 2 
90 A from 

30CFR18 
New I value 

"0" then "1" 
30CFR18 value 
New I value 

"0" then ")" 
30CFR18 value 
New I value 

Same cycle 
Vary percent on 

to "10" 
AWG #0 (same) 
30CFR18 (same) 
New I value 



5.*** A 
0.50 *** B 

66335. ***C,R/S 

90 *** D 

*** E 125. 

83656. ***C,R/S 
100. *** D 
239. *** E 

105500. ***C,R/S 
120. ***D 

***E 167. 



5, *** 

0.10 *** B 

105500. *** C 

120. *** D 

*** E 

83656. *** C 



Vary AWG # 

to "4" 
30CFR18 100. *** D 

New I value *** E 

250 pet of 30CFR18 rating 



300. 



250. 



.1 pet on 1.000000000-03 *** B 
AWG #1 83656. *** C 

30CFR18 100. *** D 

New I value *** E 250. 

(NOTE; 250 pet is maximum allowed.) 

For additional values, enter only the 
changed parameters, depress proper 
letter, then "E". 



Input Values 

A - Cycle time 

B - Percent on 

C - Cable area1 

D - 30CFR18 rating 

E - Execute 

known, enter "0" (zero) 



depress 



then enter AWG nixmber, 



81 



Conyuter BASIC Program 
AMPCAP 19-FEB-81 08:06:03 

5 DIM 12(60,10) 

6 REM CALCULATES AREA FROM WIRE SIZE OR USES AREA DIRECTLY, TO OBTAIN AVERAGE 

7 REM THERMAL TIME CONSTANT OF CONDUCTORS. 

10 PRINT "AMPACITY TABLE CALCULATION" 

11 PRINT "THERMAL RATING ONLY. VOLTAGE DROP SHOULD ALSO BE CONSIDERED." 
20 PRINT " " 

30 PRINT " " 

40 PRINT "ENTER WIRE SIZE, RATING FROM 30CFR18, AND MAX TIME IN MINUTES." 

41 PRINT "FOR SIZE 00 USE -1, FOR SIZE 000 USE -2, FOR SIZE 0000 USE -3" 
50 INPUT W,R,L 

55 R1-2.5*R 

60 IF W<250 GO TO 190 

70 A1=1000*W 

80 C=.13*SQR(A1) 

90 FOR T=l TO L 
100 FOR P=l TO 10 
110 P2=.1*P 

120 Nl=l-(2.71828-(-T/C)) 
130 Dl=l-(2.71828-(-P2*T/C)) 
140 K=SQR(N1/D1) 
150 I2(T,P)=INT(K*R) 

155 IF I2(T,P)>R1 THEN 12(T,P)=INT(R1 ) 
160 NEXT P 
170 NEXT T 
180 GO TO 240 
190 REM CALCULATES AREA 
200 Al=105500*(2.71828-( .232*W)) 
210 GO TO 80 

220 REM PRINTING FORMAT MAY BE IMPROVED BY BASIC 'PRINT USING' STATEMENT 
230 REM IF YOUR COMPUTER SUPPORTS IT. 

PERCENT OPERATING TIME" 

30";" 40";" 50";" 60";" 70";" 80' 



240 PRINT "CYCLE";" ";" 


PERCE 


245 IF R<90 GO TO 250 




246 PRINT "(MIN)";" 10";" 


20"; 


90";" 


100" 


247 GO TO 255 




250 PRINT "(MIN)";" 10";" 


20"; 


90";" 


100" 



30";" 40";" 50";" 60";" 70";" 80";' 
90";" 100" 
255 PRINT " " 
260 FOR T-1 TO L 
270 PRINT T;" ";I2(T, 1) ; I2(T,2) ; I2(T,3) ; I2(T,4) ; I2(T,5) ; I2(T,6) ; I2(T,7) ; I2(T,8) ; 

I2(T,9);I2(T,10) 
280 NEXT T 
290 PRINT " " 
300 PRINT " " 

310 PRINT "next?( Y or N )" 
320 INPUT Q$ 
330 IF Q$="Y" GO TO 20 
340 PRINT " SIGN OFF" 



82 



SEMICONDUCTING RUBBER AS A LOW-VOLTAGE SHIELD FOR PERSONNEL PROTECTION 
By J. N. Tomlinson'' and L. A. Morley2 

ABSTRACT 



Semiconducting rubber insulation is 
used in high-voltage distribution cable 
to provide a gradual transition of poten- 
tial, in order to avoid the occurrence of 
harmful corona. Metallic shielding is 
used in both distribution and trailing 
high-voltage cables to assure that any 
fault to a phase conductor will be a rel- 
atively low current phase-to-ground fault 
through the shield rather than a highly 
energetic phase-to-phase fault. A strong 
desire of users of mining machines em- 
ploying cable reels has been to find a 
way to make a cable with semiconducting 
shielding take the place of metallically 
shielded cable, which is so much heavier, 
larger, and less flexible. Research 



shows no semiconducting formulation capa- 
ble of conducting the fault current of 
several amperes that is usually consid- 
ered necessary for operations of current- 
operated ground fault protection devices. 
A voltage-sensing protection method has 
been found, however, employing an addi- 
tional conductor in the cable. This 
method is so sensitive that, as discussed 
in this paper, the circuit can be inter- 
rupted before the semiconducting rubber 
in the vicinity of a phase-to-ground 
fault is carbonized. Therefore, it 
should be possible to provide cable reel 
installations with short-circuit protec- 
tion almost equivalent to that provided 
by metallic shielding. 



INTRODUCTION 



Shielding of low-voltage trailing ca- 
bles, such as those used on shuttle cars, 
offers improved personnel safety but is 
not easily implemented simply because the 
usual shield designs cannot withstand the 
excessive flexing. Under Federal Bureau 
of Mines Contracts G01883063 and 
J0199106,4 evaluations have been made 
of shield designs using copper-copper, 
copper-cotton, and copper-nylon braid 
combinations as well as copper stranding 
with semi conductive rubber materials. 
None of those proved to be satisfactory 
mainly because of copper fatigue. How- 
ever, semi conductive rubbers are able to 
withstand the flexing and might provide a 

^Instructor of Mining Engineering, 

Pennsylvania State University, University 
Park, Pa. 

^Professor of Mining Engineering, 

Pennsylvania State University, University 
Park, Pa. 



suitable shield if materials having suf- 
ficient electrical conductivity can be 
used. This paper is based on a portion 
of the research conducted under Bureau of 
Mines contract J0199106 in which the 
authors have addressed the resistiv- 
ity requirements for conductive -rubber 
shielding materials in conjunction with 
fault-sensing equipment sensitivities and 
power-interruption times. 



Morley. Mine Trailing Cables and Ca- 
ble Splices: Shielded Cables (Contract 
60188036, Pennsylvania State Univ.). 
BuMines OFR 81-80, Feb. 29, 1980, 69 pp.; 
NTIS PB 80-208135. 

^Pennsylvania State University. Mine 
Trailing Cables and Cable Splices. 
September 1979, 109 pp.; available for 
consultation at the Bureau of Mines 
Pittsburgh Research Center, Pittsburgh, 
Pa. 



83 



THE PHYSICAL CASE 



A typical example of how shielding 
might improve personnel safety is illus- 
trated in figure 1. In this highly sim- 
plified case, a nail has pierced the ca- 
ble and made contact with an energized 
conductor. At the same instant, a person 
is in contact with the nail and also with 
the grounded machine frame. The result 
is current flow through the person as 
well as through the shield to the ground- 
ing conductor. (A similar situation 
could result if, for instance, a mechanic 
were to cut into an energized cable in 
preparation for making a splice repair. ) 
For this simplified and strictly 



resistive case, the portion of current 
through the worker is 



^" ^ (Rs + V Rg + ^sK 



(1) 



where V is the phase-to-neutral voltage, 
Rg is the resistance through the shield 
from the nail to the grounding conductor, 
R is the ohmic value of the grounding 
resistor, and R^ is the worker's effec- 
tive resistance which can vary apprecia- 
bly depending on contact resistance, body 
weight, and so forth. 



SHIELD RESISTANCE 



The situation for the nail pierc- 
ing the semiconducting sheath is shown 
in figure 2, and also in figures 3 
and 4 when the sheath is unwrapped 
from the insulated conductor to form 
a uniform layer of semiconducting 



material as illustrated. The condi- 
tion of the current flow in figure 4 
between the nail and one side of the 
grounding conductor is equivalent to 
the heat flow from a heated cylinder^ 
to an exposed wall as illustrated in 




Penetrating object 
Im — ti 



Current through shield 
Is-i- 



^ Grounding 
resistor 



Rs 

Resistance to 
grounding conductor 
(conductive rubber) 



Rm 

Resistance 
associated 
with person 



Grounding conductor 



FIGURE 1, - Parallel current paths through person and shield for a simplified circuit. 



'Incropera, F. P., and D. P. Dewitt. Fundamentals of Heat Transfer. John Wiley & 



Sons, New York, 1981, p. 140. 



84 




figure 5. The analogy for electrical re- 
sistance, R, is thus 



Nail 



Semiconducting 
shield 

FIGURE 2. - Nail piercing cable and shorting 
semiconducting shield to power 
conductor! 



Nail 




Grounding conductor 



FIGURE 3, = Shielding material unwrapped from 
insulated conductor. 



^in^:^\ 

2TrL Vd / 



(2) 



where py is the volume resistivity, L is 
the thickness of the semiconducting 
layer, h is the distance from the center 
of the cylindrical probe (or nail) to the 
grounding conductor, and D is the probe 
diameter. 

Empirical and theoretical values of 
the resistance, R, have been obtained and 
are plotted in figure 6 as a function of 
h where h is given in terms of probe 
diameters. The difference between the 
two curves is attributed to the finite 
limits used in the experimental eval- 
uations. The curves in the figure have 
been normalized for p^ =1 fl-cm and L 
= 1 cm. 

Since the piercing object will in 
effect be somewhere between two grounding 
conductors (fig. 3), the resistance is 
more like that shown in figure 7 and a 
value on the order of 6 ^ might be ap- 
proximated if Py were equal to 1 fi-cm and 
the shield thickness, L, were on the 
order of 1 mm. 

MAXIMUM CURRENTS VERSUS TIME 

Values for I^ from equation 1 are 
plotted in figure 8 for three different 
voltages. 



Nail 




FIGURE 4. - Shielding further simplified for 
analysis of current between pow- 
er and grounding conductors. 



The concern in electrical shock is that 
the victim's heart will lose its normal 
rhythm and go into ventricular fibrilla- 
tion. The current level and time expo- 
sure at which this statistically occurs 
for a typical human has been given by 
DalzielS as 



I = 



116 



where 
amp 



I is in minimum current 
and t is duration of 



in milli- 
shock in 



^Dalziel, D. F. Electric Shock Hazard. 
IEEE Spectrum, February 1973, pp. 44-50. 



85 



Piercing object 




^°h 



Semiconducting 
sliielding 



FIGURES. 



Electrical path from piercing object to 
grounding conductor via the semicon- 
ducting shield material. 





Grounding 
conductor 



Theoretical 

= A_ ln(4h/D) 

''v =1 -TL cm 
L =1 cm 



■ 20 40 60 80 100 120 140 160 

DISTANCE FROM PROBE TO GROUNDING 

CONDUCTOR, probe diameters 

FIGURE 6. - Theoretical versus experimental resist- 
ances between probe and grounding con- 
ductor in a conductive medium. 



seconds. The values of t are scaled 
against I^ in figure 8 and provide an 
indication of the time requirements for 



power interruption in order that the 
shield is effective for the operating 
conditions considered. 



VOLUME-RESISTIVITY TEST DATA 



Volume-resistivity measurements were 
made on semiconducting shield materials 
used in three prototype cable designs and 
another cable used in a similar mining 
application in the United Kingdom. Also, 
measurements were made on the conductive 



material used in a high-voltage cable 
application, and some semiconducting tape 
was also evaluated. The sample config- 
urations are illustrated in figures 9 and 
10. Table 1 presents the laboratory 
values. 



TABLE 1. - Volume resistivity values for various 
semiconducting material samples 





Typical volume resistivities (fi-cm) 


Sample 


Along extrusion 
direction 


Across extrusion 
direction 


Prototype 1 

Prototype 2 

Prototype 3 

Drill cord 


750 
600 
280 
300 
12,500 
240 


415 
500 
180 
130 


High-voltage cable. . . 
Semiconducting tape. . 


5,600 
610 



86 



1.0 



CJ 0.5 



LU 
O 



Empirical 



Grounding 
conductor 




pv = 1 -TL-cm 
L=1 cm 







20 40 60 80 



DISTANCE FROM PROBE TO 

GROUNDING CONDUCTOR, 

probe diameters 

FIGURE 7. = Resistances with probe placed be- 
tween two conductors. 

Applying some of the values from ta- 
ble 1 to the curves in figure 8, it is 



obvious that power-interruption times 
would have to be extremely short for 
the high system voltages where the 
maximum ground-fault current is limited 
to 25 A. If R was set to limit maximum 
ground-fault current to 750 mA, the re- 
quired power-interruption times might 
be more practical, especially in the 
low-voltage cases. Materials with vol- 
ume resistivities orders of magnitude 
lower than the best values listed in 
table 1 would, of course, provide a much 
better safety margin for the shield 
design. 

The advantage of higher-ohmage ground- 
ing resistors is quite obvious from the 
curves presented in figure 8. The higher 
values also limit ground-fault current in 
the shield which might otherwise volatize 
the shield material and provide no shunt 
protection for the victim. This latter 
advantage is also true should copper 
braid bemused rather than semiconducting 
materials. 

The curves in figure 8 are generated 
using a resistance value of 500 SI for the 
victim. Volume resistivities of 100 fi-cm 
would produce about equal current flow 
through the shield and through the vic- 
tim, all other resistances being equal. 
It is also important to note that while 
the shield is actually being cut into, 
the contact resistance is much higher, 
and this would reduce the initial shunt- 
ing effect even more. 



FAULT-TEST EXPERIMENTS 



Numerous tests were conducted in 
which a nail was driven into proto- 
type cables having semiconducting rub- 
ber shields of the SHD configuration. 
Both ac, 295-V line-to-neutral and dc, 
245-V line-to-neutral systems were used. 



Grounding resistors limited the maximum 
current to 15 A. Fault sensing used a 
zero-sequence method for the ac system 
and saturable-reactor sensing for the dc 
system. 



87 



1,000 



500 



100 



1 50 

o 

> 

I 
o 

o 

I 



10 



5 - 







^^^^ 


- 


950 V- 


^V^ 


- 




7 


- 


480 V-^ 


/^^^ 


- 


208 V-^vV/ 


950 V-^^.y/|^^^'^ 


^^^^^^^ 


_ 




f/^-AQO V 


- 




M. 


- 




1/ ^^^208 V 


~ 




/ 


- 




/ ^^^-lm = 750mA 






^^— lm-25A 


- / 




Rm = 500^ 


- 


/ 1 


1 I 



25 S 



LU 

50 2 



100 



^ 250 3 

_j 

500 CD 
LL 

1,000 ir 
< 

-J 

2,500 O 
5,000 z 

LU 
> 



0.1 



1.0 



10 



100 1,000 



VOLUME RESISTIVITY OF SHIELD MATERIAL, ii-cm 

FIGURE 8. - Currents and ventricular fibrillation times versus shield resistivities for 
various operating voltages and ground resistor. 



88 



Copper paint 




Circumferential 
test sample 



FIGURE 9. - Sample configurations taken from semi- 
conducting shielded cable used to de- 
termine volume resistivity, pv. 



Razor blade 




FIGURE 10. - Sample configurations used in measur- 
ing pv for semiconducting tape samples. 



The results were in two general cate- 
gories. For the first group, the time 
required to interrupt the power ranged 
from slightly less than 1 sec to as high 
as 2.5 sec, depending on sensing method, 
puncture location, and so forth. The 
resistance between the nail and the 
grounding conductor was time dependent 
and decreased as the I^r heated the mate- 
rial. In most cases, the semiconducting 
material actually carbonized before the 
power was interrupted. In the second 
group, the immediate current flow vola- 
tized the semiconducting shield material 
in contact with the nail, and the power 
was not interrupted at all. Approxi- 
mately 1 out of every 10 experiments pro- 
duced this result. 

The second group in which the shield 
was volatized obviously provided no 



personnel protection. However, a higher 
grounding resistance would limit the 
current to a less destructive value and 
so provide an improvement for these 
cases. 

A higher grounding resistance for the 
first group would tend to lengthen the 
already-too-long time between initial 
contact and power interruption since the 
i2r dissipated in the semiconducting 
shield would decrease; thus, the rate of 
resistance change between the nail and 
the grounding conductor would be les- 
sened. The solution here is high- 
resistance grounding along with lew- 
resistivity shielding materials. Safety 
limitations would simply be the current- 
sensing abilities of the ground-fault 
system and associated power-interniption 
delays. 



CONCLUSIONS 



The results of this work suggest 
the semiconducting shielding might 
have application in low-voltage trail- 
ing cables under the following 
circumstances. 



(2) High-ohmage grounding resistors are 
required to allow more time to sense and 
interrupt the power and to limit current 
levels which might otherwise volatize the 
the shield material. 



(1) Volume resistivities on the order 
of 1 fi-cm must be applied to the cables 
of interest. It is important to note 
that volume resistivities measured on 
laboratory slab samples are usually 
much lower than that measured for sam- 
plings taken from an actual extruded 
shield. 

1982 - 505 - 002/70 



(3) High-sensitivity ground-fault de- 
tectors are needed. Combining semi- 
conductive shielding of trailing cables, 
very low ground-current limits, and high- 
sensitivity ground-fault relaying should 
substantially reduce electrocution haz- 
zards on portable or mobile mining 
equipment. 

INT.-BU.OF MINES, PGH., PA. 26334 



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